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-command/src/help/help_aliases.rs

@@ -1,5 +1,5 @@
 use crate::filters::find_internal;
-use nu_engine::{command_prelude::*, scope::ScopeData};
+use nu_engine::{command_prelude::*, get_full_help, scope::ScopeData};
 
 #[derive(Clone)]
 pub struct HelpAliases;
@@ -101,42 +101,17 @@ pub fn help_aliases(
             });
         };
 
-        let Some(alias) = engine_state.get_decl(alias).as_alias() else {
+        let alias = engine_state.get_decl(alias);
+
+        if alias.as_alias().is_none() {
             return Err(ShellError::AliasNotFound {
                 span: Span::merge_many(rest.iter().map(|s| s.span)),
             });
         };
 
-        let alias_expansion =
-            String::from_utf8_lossy(engine_state.get_span_contents(alias.wrapped_call.span));
-        let description = alias.description();
-        let extra_desc = alias.extra_description();
+        let help = get_full_help(alias, engine_state, stack);
 
-        // TODO: merge this into documentation.rs at some point
-        const G: &str = "\x1b[32m"; // green
-        const C: &str = "\x1b[36m"; // cyan
-        const RESET: &str = "\x1b[0m"; // reset
-
-        let mut long_desc = String::new();
-
-        long_desc.push_str(description);
-        long_desc.push_str("\n\n");
-
-        if !extra_desc.is_empty() {
-            long_desc.push_str(extra_desc);
-            long_desc.push_str("\n\n");
-        }
-
-        long_desc.push_str(&format!("{G}Alias{RESET}: {C}{name}{RESET}"));
-        long_desc.push_str("\n\n");
-        long_desc.push_str(&format!("{G}Expansion{RESET}:\n  {alias_expansion}"));
-
-        let config = stack.get_config(engine_state);
-        if !config.use_ansi_coloring.get(engine_state) {
-            long_desc = nu_utils::strip_ansi_string_likely(long_desc);
-        }
-
-        Ok(Value::string(long_desc, call.head).into_pipeline_data())
+        Ok(Value::string(help, call.head).into_pipeline_data())
     }
 }