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-test-support/src/macros.rs

@@ -202,11 +202,11 @@ macro_rules! nu_with_std {
 
 #[macro_export]
 macro_rules! nu_with_plugins {
-    (cwd: $cwd:expr, plugins: [$(($plugin_name:expr)),+$(,)?], $command:expr) => {{
+    (cwd: $cwd:expr, plugins: [$(($plugin_name:expr)),*$(,)?], $command:expr) => {{
         nu_with_plugins!(
             cwd: $cwd,
             envs: Vec::<(&str, &str)>::new(),
-            plugins: [$(($plugin_name)),+],
+            plugins: [$(($plugin_name)),*],
             $command
         )
     }};
@@ -222,10 +222,10 @@ macro_rules! nu_with_plugins {
     (
         cwd: $cwd:expr,
         envs: $envs:expr,
-        plugins: [$(($plugin_name:expr)),+$(,)?],
+        plugins: [$(($plugin_name:expr)),*$(,)?],
         $command:expr
     ) => {{
-        $crate::macros::nu_with_plugin_run_test($cwd, $envs, &[$($plugin_name),+], $command)
+        $crate::macros::nu_with_plugin_run_test($cwd, $envs, &[$($plugin_name),*], $command)
     }};
     (cwd: $cwd:expr, envs: $envs:expr, plugin: ($plugin_name:expr), $command:expr) => {{
         $crate::macros::nu_with_plugin_run_test($cwd, $envs, &[$plugin_name], $command)
@@ -329,12 +329,14 @@ where
     });
 
     let temp = tempdir().expect("couldn't create a temporary directory");
-    let [temp_config_file, temp_env_config_file, temp_plugin_file] =
-        ["config.nu", "env.nu", "plugin.nu"].map(|name| {
-            let temp_file = temp.path().join(name);
-            std::fs::File::create(&temp_file).expect("couldn't create temporary config file");
-            temp_file
-        });
+    let [temp_config_file, temp_env_config_file] = ["config.nu", "env.nu"].map(|name| {
+        let temp_file = temp.path().join(name);
+        std::fs::File::create(&temp_file).expect("couldn't create temporary config file");
+        temp_file
+    });
+
+    // We don't have to write the plugin cache file, it's ok for it to not exist
+    let temp_plugin_file = temp.path().join("plugin.msgpackz");
 
     crate::commands::ensure_plugins_built();