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-parser/src/parse_keywords.rs

@@ -3550,7 +3550,8 @@ pub fn parse_where(working_set: &mut StateWorkingSet, lite_command: &LiteCommand
 pub fn parse_register(working_set: &mut StateWorkingSet, lite_command: &LiteCommand) -> Pipeline {
     use nu_plugin::{get_signature, PersistentPlugin, PluginDeclaration};
     use nu_protocol::{
-        engine::Stack, IntoSpanned, PluginIdentity, PluginSignature, RegisteredPlugin,
+        engine::Stack, IntoSpanned, PluginCacheItem, PluginIdentity, PluginSignature,
+        RegisteredPlugin,
     };
 
     let spans = &lite_command.parts;
@@ -3743,10 +3744,10 @@ pub fn parse_register(working_set: &mut StateWorkingSet, lite_command: &LiteComm
                     )
                 });
 
-                if signatures.is_ok() {
-                    // mark plugins file as dirty only when the user is registering plugins
-                    // and not when we evaluate plugin.nu on shell startup
-                    working_set.mark_plugins_file_dirty();
+                if let Ok(ref signatures) = signatures {
+                    // Add the loaded plugin to the delta
+                    working_set
+                        .update_plugin_cache(PluginCacheItem::new(&identity, signatures.clone()));
                 }
 
                 signatures