Overhaul the plugin cache file with a new msgpack+brotli format (#12579)

# Description

- Plugin signatures are now saved to `plugin.msgpackz`, which is
brotli-compressed MessagePack.
- The file is updated incrementally, rather than writing all plugin
commands in the engine every time.
- The file always contains the result of the `Signature` call to the
plugin, even if commands were removed.
- Invalid data for a particular plugin just causes an error to be
reported, but the rest of the plugins can still be parsed

# User-Facing Changes

- The plugin file has a different filename, and it's not a nushell
script.
- The default `plugin.nu` file will be automatically migrated the first
time, but not other plugin config files.
- We don't currently provide any utilities that could help edit this
file, beyond `plugin add` and `plugin rm`
  - `from msgpackz`, `to msgpackz` could also help
- New commands: `plugin add`, `plugin rm`

# Tests + Formatting

Tests added for the format and for the invalid handling.

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

# After Submitting

- [ ] Check for documentation changes
- [ ] Definitely needs release notes

src/config_files.rs

@@ -214,7 +214,7 @@ pub(crate) fn setup_config(
     );
     let result = catch_unwind(AssertUnwindSafe(|| {
         #[cfg(feature = "plugin")]
-        read_plugin_file(engine_state, stack, plugin_file, NUSHELL_FOLDER);
+        read_plugin_file(engine_state, plugin_file, NUSHELL_FOLDER);
 
         read_config_file(engine_state, stack, env_file, true);
         read_config_file(engine_state, stack, config_file, false);