remove the deprecated register command (#13297)

# Description

This PR removes the `register` command which has been
[deprecated](https://www.nushell.sh/blog/2024-04-30-nushell_0_93_0.html#register-toc)
in favor of [`plugin
add`](https://www.nushell.sh/blog/2024-04-30-nushell_0_93_0.html#redesigned-plugin-management-commands-toc)

# User-Facing Changes

`register` is no longer available

crates/nu-protocol/src/config/mod.rs

@@ -104,7 +104,7 @@ pub struct Config {
     /// Configuration for plugins.
     ///
     /// Users can provide configuration for a plugin through this entry.  The entry name must
-    /// match the registered plugin name so `register nu_plugin_example` will be able to place
+    /// match the registered plugin name so `plugin add nu_plugin_example` will be able to place
     /// its configuration under a `nu_plugin_example` column.
     pub plugins: HashMap<String, Value>,
     /// Configuration for plugin garbage collection.

crates/nu-protocol/src/engine/engine_state.rs

@@ -741,8 +741,8 @@ impl EngineState {
 
     /// Fetch the configuration for a plugin
     ///
-    /// The `plugin` must match the registered name of a plugin.  For `register nu_plugin_example`
-    /// the plugin name to use will be `"example"`
+    /// The `plugin` must match the registered name of a plugin.  For `plugin add
+    /// nu_plugin_example` the plugin name to use will be `"example"`
     pub fn get_plugin_config(&self, plugin: &str) -> Option<&Value> {
         self.config.plugins.get(plugin)
     }

crates/nu-protocol/src/plugin/registered.rs

@@ -26,7 +26,7 @@ pub trait RegisteredPlugin: Send + Sync {
     fn stop(&self) -> Result<(), ShellError>;
 
     /// Stop the plugin and reset any state so that we don't make any assumptions about the plugin
-    /// next time it launches. This is used on `register`.
+    /// next time it launches. This is used on `plugin add`.
     fn reset(&self) -> Result<(), ShellError>;
 
     /// Cast the pointer to an [`Any`] so that its concrete type can be retrieved.