Parameter defaults to $nu.scope.commands (#9152)

(*third* try at posting this PR, #9104, like #9084, got polluted with
unrelated commits. I'm never going to pull from the github feature
branch again!)

# Description
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Show parameter defaults in scope command signature, where they're
available for display by help.
per https://github.com/nushell/nushell/issues/8928.

I found unexpected ramifications in one completer (NuHelpCompleter) and
plugins, which both use the flag-formatting routine from builtin help.
For the moment I made the minimum necessary changes to get the mainline
scenario to pass tests and run. But we should circle back on what to do
with plugins and help completer..

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
1. New `parameter_default` column to `signatures` table in
`$nu.scope.commands`
It is populated with whatever parameters can be defaulted: currently
positional args and named flags.
2. Built in help (both `help <command>` and `<command> --help` will
display the defaults
3. Help completer will display defaults for flags, but not for
positionals.

Example:
A custom command with some default parameters:
```
〉cat ~/work/dflts.nu 
# sample function to show defaults in help
export def main [
    arg1: string        # mandatory positional
    arg2:string=abc     # optional positional
    --switch            # no default here
    --named:int         # named flag, no default
    --other:string=def  # flag 
    --hard:record<foo:int bar:string, bas:bool> # default can be compound type
            = {foo:22, bar:"other worlds", bas:false}
] { {arg1: $arg1,
    arg2: $arg2,
    switch: $switch,
    named: $named,
    other: $other,
    hard: $hard, }
}

〉use ~/work/dflts.nu

〉$nu.scope.commands | where name == 'dflts' | get signatures.0.any | reject short_flag description custom_completion
╭───┬────────────────┬────────────────┬──────────────────────────────────────────┬─────────────┬───────────────────────────╮
│ # │ parameter_name │ parameter_type │               syntax_shape               │ is_optional │     parameter_default     │
├───┼────────────────┼────────────────┼──────────────────────────────────────────┼─────────────┼───────────────────────────┤
│ 0 │                │ input          │ any                                      │ false       │                           │
│ 1 │ arg1           │ positional     │ string                                   │ false       │                           │
│ 2 │ arg2           │ positional     │ string                                   │ true        │ abc                       │
│ 3 │ switch         │ switch         │                                          │ true        │                           │
│ 4 │ named          │ named          │ int                                      │ true        │                           │
│ 5 │ other          │ named          │ string                                   │ true        │ def                       │
│ 6 │ hard           │ named          │ record<foo: int, bar: string, bas: bool> │ true        │ ╭───────┬───────────────╮ │
│   │                │                │                                          │             │ │ foo   │ 22            │ │
│   │                │                │                                          │             │ │ bar   │ other worlds  │ │
│   │                │                │                                          │             │ │ bas   │ false         │ │
│   │                │                │                                          │             │ ╰───────┴───────────────╯ │
│ 7 │                │ output         │ any                                      │ false       │                           │
╰───┴────────────────┴────────────────┴──────────────────────────────────────────┴─────────────┴───────────────────────────╯

〉help dflts
sample function to show defaults in help

Usage:
  > dflts {flags} <arg1> (arg2) 

Flags:
  --switch - switch -- no default here
  --named <Int> - named flag, typed, but no default
  --other <String> - flag with default (default: 'def')
  --hard <Record([("foo", Int), ("bar", String), ("bas", Boolean)])> - default can be compound type (default: {foo: 22, bar: 'other worlds', bas: false})
  -h, --help - Display the help message for this command

Parameters:
  arg1 <string>: mandatory positional
  arg2 <string>: optional positional (optional, default: 'abc')
```

Compared to (relevant bits of) help output previously:
```
Flags:
  -h, --help - Display the help message for this command
  -, --switch - no default here
  -, --named <int> - named flag, no default
  -, --other <string> - flag
  -, --hard <record<foo: int, bar: string, bas: bool>> - default can be compound type

Signatures:
  <any> | dflts <string> <string> -> <any>

Parameters:
  arg1 <string>: mandatory positional
  (optional) arg2 <string>: optional positional
```

# Tests + Formatting
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# After Submitting
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crates/nu-protocol/src/value/mod.rs

@@ -643,6 +643,36 @@ impl Value {
         format!("{self:#?}")
     }
 
+    /// Convert Value into a parsable string (quote strings)
+    /// bugbug other, rarer types not handled
+
+    pub fn into_string_parsable(&self, separator: &str, config: &Config) -> String {
+        match self {
+            // give special treatment to the simple types to make them parsable
+            Value::String { val, .. } => format!("'{}'", val),
+
+            // recurse back into this function for recursive formatting
+            Value::List { vals: val, .. } => format!(
+                "[{}]",
+                val.iter()
+                    .map(|x| x.into_string_parsable(", ", config))
+                    .collect::<Vec<_>>()
+                    .join(separator)
+            ),
+            Value::Record { cols, vals, .. } => format!(
+                "{{{}}}",
+                cols.iter()
+                    .zip(vals.iter())
+                    .map(|(x, y)| format!("{}: {}", x, y.into_string_parsable(", ", config)))
+                    .collect::<Vec<_>>()
+                    .join(separator)
+            ),
+
+            // defer to standard handling for types where standard representation is parsable
+            _ => self.into_string(separator, config),
+        }
+    }
+
     /// Convert Value into string. Note that Streams will be consumed.
     pub fn debug_string(&self, separator: &str, config: &Config) -> String {
         match self {