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-cmd-plugin/src/default_context.rs

@@ -0,0 +1,31 @@
+use crate::*;
+use nu_protocol::engine::{EngineState, StateWorkingSet};
+
+pub fn add_plugin_command_context(mut engine_state: EngineState) -> EngineState {
+    let delta = {
+        let mut working_set = StateWorkingSet::new(&engine_state);
+
+        macro_rules! bind_command {
+            ( $( $command:expr ),* $(,)? ) => {
+                $( working_set.add_decl(Box::new($command)); )*
+            };
+        }
+
+        bind_command!(
+            PluginCommand,
+            PluginAdd,
+            PluginList,
+            PluginRm,
+            PluginStop,
+            Register,
+        );
+
+        working_set.render()
+    };
+
+    if let Err(err) = engine_state.merge_delta(delta) {
+        eprintln!("Error creating default context: {err:?}");
+    }
+
+    engine_state
+}