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-plugin/src/plugin/mod.rs

@@ -9,7 +9,7 @@ use std::{
     thread,
 };
 
-use nu_engine::documentation::get_flags_section;
+use nu_engine::documentation::{get_flags_section, HelpStyle};
 use nu_plugin_core::{
     ClientCommunicationIo, CommunicationMode, InterfaceManager, PluginEncoder, PluginRead,
     PluginWrite,
@@ -657,6 +657,7 @@ fn print_help(plugin: &impl Plugin, encoder: impl PluginEncoder) {
     println!("Encoder: {}", encoder.name());
 
     let mut help = String::new();
+    let help_style = HelpStyle::default();
 
     plugin.commands().into_iter().for_each(|command| {
         let signature = command.signature();
@@ -670,7 +671,7 @@ fn print_help(plugin: &impl Plugin, encoder: impl PluginEncoder) {
                 }
             })
             .and_then(|_| {
-                let flags = get_flags_section(None, None, &signature, |v| format!("{:#?}", v));
+                let flags = get_flags_section(&signature, &help_style, |v| format!("{:#?}", v));
                 write!(help, "{flags}")
             })
             .and_then(|_| writeln!(help, "\nParameters:"))