Deprecate register and add plugin use (#12607)

# Description

Adds a new keyword, `plugin use`. Unlike `register`, this merely loads
the signatures from the plugin cache file. The file is configurable with
the `--plugin-config` option either to `nu` or to `plugin use` itself,
just like the other `plugin` family of commands. At the REPL, one might
do this to replace `register`:

```nushell
> plugin add ~/.cargo/bin/nu_plugin_foo
> plugin use foo
```

This will not work in a script, because `plugin use` is a keyword and
`plugin add` does not evaluate at parse time (intentionally). This means
we no longer run random binaries during parse.

The `--plugins` option has been added to allow running `nu` with certain
plugins in one step. This is used especially for the `nu_with_plugins!`
test macro, but I'd imagine is generally useful. The only weird quirk is
that it has to be a list, and we don't really do this for any of our
other CLI args at the moment.

`register` now prints a deprecation parse warning.

This should fix #11923, as we now have a complete alternative to
`register`.

# User-Facing Changes

- Add `plugin use` command
- Deprecate `register`
- Add `--plugins` option to `nu` to replace a common use of `register`

# Tests + Formatting

I think I've tested it thoroughly enough and every existing test passes.
Testing nu CLI options and alternate config files is a little hairy and
I wish there were some more generic helpers for this, so this will go on
my TODO list for refactoring.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

- [ ] Update plugins sections of book
- [ ] Release notes

crates/nu-parser/src/parser.rs

@@ -5149,12 +5149,24 @@ pub fn parse_expression(working_set: &mut StateWorkingSet, spans: &[Span]) -> Ex
             #[cfg(feature = "plugin")]
             b"register" => {
                 working_set.error(ParseError::BuiltinCommandInPipeline(
-                    "plugin".into(),
+                    "register".into(),
                     spans[0],
                 ));
 
                 parse_call(working_set, &spans[pos..], spans[0])
             }
+            #[cfg(feature = "plugin")]
+            b"plugin" => {
+                if spans.len() > 1 && working_set.get_span_contents(spans[1]) == b"use" {
+                    // only 'plugin use' is banned
+                    working_set.error(ParseError::BuiltinCommandInPipeline(
+                        "plugin use".into(),
+                        spans[0],
+                    ));
+                }
+
+                parse_call(working_set, &spans[pos..], spans[0])
+            }
 
             _ => parse_call(working_set, &spans[pos..], spans[0]),
         }
@@ -5286,6 +5298,20 @@ pub fn parse_builtin_commands(
         b"where" => parse_where(working_set, lite_command),
         #[cfg(feature = "plugin")]
         b"register" => parse_register(working_set, lite_command),
+        // Only "plugin use" is a keyword
+        #[cfg(feature = "plugin")]
+        b"plugin"
+            if lite_command
+                .parts
+                .get(1)
+                .is_some_and(|span| working_set.get_span_contents(*span) == b"use") =>
+        {
+            if let Some(redirection) = lite_command.redirection.as_ref() {
+                working_set.error(redirecting_builtin_error("plugin use", redirection));
+                return garbage_pipeline(&lite_command.parts);
+            }
+            parse_keyword(working_set, lite_command)
+        }
         _ => {
             let element = parse_pipeline_element(working_set, lite_command);