Rework help generation internals (#13531)

Reworking some of the sprawling code we use to generate the `help cmd`
or `cmd --help` output.

This touches mainly the rendering and not the gathering of the necessary
data (see open bugs under
[label:help-system](https://github.com/nushell/nushell/issues?q=sort%3Aupdated-desc+is%3Aopen+label%3Ahelp-system))

Fixes #9076 
Fixes the syntax shape output on flags to be consistent.

## Example
```nushell
def test [
  positional: int,
  documented: float, # this has documentation
  default = 50,
  optional?,
  --a = "bla",
  --bla (-b) = "bla", # named with default
] {}
```

### before

![grafik](https://github.com/user-attachments/assets/1867984f-1289-4ad0-bdf5-c49ec56dfddb)


### after


![grafik](https://github.com/user-attachments/assets/8fca526f-d878-4d52-b970-fc41c7e8859c)

crates/nu-cli/src/menus/help_completions.rs

@@ -1,4 +1,4 @@
-use nu_engine::documentation::get_flags_section;
+use nu_engine::documentation::{get_flags_section, HelpStyle};
 use nu_protocol::{engine::EngineState, levenshtein_distance, Config};
 use nu_utils::IgnoreCaseExt;
 use reedline::{Completer, Suggestion};
@@ -20,6 +20,9 @@ impl NuHelpCompleter {
     fn completion_helper(&self, line: &str, pos: usize) -> Vec<Suggestion> {
         let folded_line = line.to_folded_case();
 
+        let mut help_style = HelpStyle::default();
+        help_style.update_from_config(&self.engine_state, &self.config);
+
         let mut commands = self
             .engine_state
             .get_decls_sorted(false)
@@ -60,12 +63,9 @@ impl NuHelpCompleter {
                 let _ = write!(long_desc, "Usage:\r\n  > {}\r\n", sig.call_signature());
 
                 if !sig.named.is_empty() {
-                    long_desc.push_str(&get_flags_section(
-                        Some(&self.engine_state),
-                        Some(&self.config),
-                        &sig,
-                        |v| v.to_parsable_string(", ", &self.config),
-                    ))
+                    long_desc.push_str(&get_flags_section(&sig, &help_style, |v| {
+                        v.to_parsable_string(", ", &self.config)
+                    }))
                 }
 
                 if !sig.required_positional.is_empty()