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

crates/nu-protocol/src/errors/shell_error.rs

@@ -750,6 +750,19 @@ pub enum ShellError {
         span: Span,
     },
 
+    /// The cached plugin data (in `$nu.plugin-path`) for a plugin is invalid.
+    ///
+    /// ## Resolution
+    ///
+    /// `register` the plugin again to update the data, or remove it.
+    #[error("The cached plugin data for `{plugin_name}` is invalid")]
+    #[diagnostic(code(nu::shell::plugin_cache_data_invalid))]
+    PluginCacheDataInvalid {
+        plugin_name: String,
+        #[help("try registering the plugin again with `{}`")]
+        register_command: String,
+    },
+
     /// A plugin failed to load.
     ///
     /// ## Resolution