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-plugin/src/lib.rs

@@ -78,10 +78,11 @@ pub use serializers::{json::JsonSerializer, msgpack::MsgPackSerializer};
 // Used by other nu crates.
 #[doc(hidden)]
 pub use plugin::{
-    create_plugin_signature, get_signature, load_plugin_cache_item, load_plugin_file,
-    serve_plugin_io, EngineInterfaceManager, GetPlugin, Interface, InterfaceManager,
-    PersistentPlugin, PluginDeclaration, PluginExecutionCommandContext, PluginExecutionContext,
-    PluginInterface, PluginInterfaceManager, PluginSource, ServePluginError,
+    add_plugin_to_working_set, create_plugin_signature, get_signature, load_plugin_cache_item,
+    load_plugin_file, serve_plugin_io, EngineInterfaceManager, GetPlugin, Interface,
+    InterfaceManager, PersistentPlugin, PluginDeclaration, PluginExecutionCommandContext,
+    PluginExecutionContext, PluginInterface, PluginInterfaceManager, PluginSource,
+    ServePluginError,
 };
 #[doc(hidden)]
 pub use protocol::{PluginCustomValue, PluginInput, PluginOutput};