Fix panic on too few arguments for custom function (#10395)

# Description
Old code was comparing remaining positional arguments with total number
of arguments, where it should've compared remaining positional with
with remaining arguments of any kind. This means that if a function was
given too few arguments, `calculate_end_span` would believe that it
actually had too many arguments, since after parsing the first few
arguments, the number of remaining arguments needed were fewer than the
*total* number of arguments, of which we had used several.

Fixes #9072
Fixes: https://github.com/nushell/nushell/issues/13930
Fixes: https://github.com/nushell/nushell/issues/12069
Fixes: https://github.com/nushell/nushell/issues/8385

Extracted from #10381

## Bonus

It also improves the error handling on missing positional arguments
before keywords (no longer crashing since #9851). Instead of just giving
the keyword to the parser for the missing positional, we give an
explicit error about a missing positional argument. I would like better
descriptions than "missing var_name" though, but I'm not sure if that's
available without

Old error
```
Error: nu::parser::parse_mismatch

  × Parse mismatch during operation.
   ╭─[entry #1:1:1]
 1 │ let = if foo
   ·     ┬
   ·     ╰── expected valid variable name
   ╰────
```

New error
```
Error: nu::parser::missing_positional

  × Missing required positional argument.
   ╭─[entry #18:1:1]
 1 │ let = foo
   ·    ┬
   ·    ╰── missing var_name
   ╰────
  help: Usage: let <var_name> = <initial_value>
```

# User-Facing Changes
The program `alias = = =` is no longer accepted by the parser

crates/nu-protocol/src/signature.rs

@@ -522,27 +522,6 @@ impl Signature {
         total
     }
 
-    pub fn num_positionals_after(&self, idx: usize) -> usize {
-        let mut total = 0;
-
-        for (curr, positional) in self.required_positional.iter().enumerate() {
-            match positional.shape {
-                SyntaxShape::Keyword(..) => {
-                    // Keywords have a required argument, so account for that
-                    if curr > idx {
-                        total += 2;
-                    }
-                }
-                _ => {
-                    if curr > idx {
-                        total += 1;
-                    }
-                }
-            }
-        }
-        total
-    }
-
     /// Find the matching long flag
     pub fn get_long_flag(&self, name: &str) -> Option<Flag> {
         for flag in &self.named {