Add backtick code formatting to help (#15892)

# Description
Adds formatting for code in backticks in `help` output. If it's possible
to highlight syntax (`nu-highlight` is available and there's no invalid
syntax) then it's highlighted. If the syntax is invalid or not an
internal command, then it's dimmed and italicized. like some of the
output from `std/help`. If `use_ansi_coloring` is `false`, then we leave
the backticks alone. Here's a couple examples:


![image](https://github.com/user-attachments/assets/57eed1dd-b38c-48ef-92c6-3f805392487c)


![image](https://github.com/user-attachments/assets/a0efa0d7-fc11-4702-973b-a0b448c383e0)

(note on this one: usually we can highlight partial commands, like `get`
in the `select` help page which is invalid according to `nu-check` but
is still properly highlighted, however `where` is special cased and just
typing `where` with no row condition is highlighted with the garbage
style so `where` alone isn't highlighted here)

![image](https://github.com/user-attachments/assets/28c110c9-16c4-4890-bc74-6de0f2e6d1b8)

here's the `where` page with `$env.config.use_ansi_coloring = false`:

![image](https://github.com/user-attachments/assets/57871cc8-d509-4719-9dd4-e6f24f9d891c)


Technically, some syntax is valid but isn't really "Nushell code". For
example, the `select` help page has a line that says "Select just the
\`name\` column". If you just type `name` in the REPL, Nushell treats it
as an external command, but for the purposes of highlighted we actually
want this to fall back to the generic dimmed/italic style. This is
accomplished by temporarily setting the `shape_external` and
`shape_externalarg` color config to the generic/fallback style, and then
restoring the color config after highlighting. This is a bit hack-ish
but it seems to work pretty well.


# User-Facing Changes

- `help` command now supports code backtick formatting. Code will be
highlighted using `nu-highlight` if possible, otherwise it will fall
back to a generic format.
- Adds `--reject-garbage` flag to `nu-highlight` which will return an
error on invalid syntax (which would otherwise be highlighted with
`$env.config.color_config.shape_garbage`)

# Tests + Formatting

Added tests for the regex. I don't think tests for the actual
highlighting are very necessary since the failure mode is graceful and
it would be difficult to meaningfully test.

# After Submitting

N/A

---------

Co-authored-by: Piepmatz <git+github@cptpiepmatz.de>

crates/nu-plugin/src/plugin/mod.rs

@@ -9,7 +9,7 @@ use std::{
     thread,
 };
 
-use nu_engine::documentation::{HelpStyle, get_flags_section};
+use nu_engine::documentation::{FormatterValue, HelpStyle, get_flags_section};
 use nu_plugin_core::{
     ClientCommunicationIo, CommunicationMode, InterfaceManager, PluginEncoder, PluginRead,
     PluginWrite,
@@ -684,7 +684,10 @@ fn print_help(plugin: &impl Plugin, encoder: impl PluginEncoder) {
                 }
             })
             .and_then(|_| {
-                let flags = get_flags_section(&signature, &help_style, |v| format!("{:#?}", v));
+                let flags = get_flags_section(&signature, &help_style, |v| match v {
+                    FormatterValue::DefaultValue(value) => format!("{:#?}", value),
+                    FormatterValue::CodeString(text) => text.to_string(),
+                });
                 write!(help, "{flags}")
             })
             .and_then(|_| writeln!(help, "\nParameters:"))