Change PluginCommand API to be more like Command (#12279)

# Description

This is something that was discussed in the core team meeting last
Wednesday. @ayax79 is building `nu-plugin-polars` with all of the
dataframe commands into a plugin, and there are a lot of them, so it
would help to make the API more similar. At the same time, I think the
`Command` API is just better anyway. I don't think the difference is
justified, and the types for core commands have the benefit of requiring
less `.into()` because they often don't own their data

- Broke `signature()` up into `name()`, `usage()`, `extra_usage()`,
`search_terms()`, `examples()`
- `signature()` returns `nu_protocol::Signature`
- `examples()` returns `Vec<nu_protocol::Example>`
- `PluginSignature` and `PluginExample` no longer need to be used by
plugin developers

# User-Facing Changes
Breaking API for plugins yet again 😄

crates/nu_plugin_example/src/lib.rs

@@ -4,9 +4,9 @@ mod commands;
 mod example;
 
 pub use commands::*;
-pub use example::Example;
+pub use example::ExamplePlugin;
 
-impl Plugin for Example {
+impl Plugin for ExamplePlugin {
     fn commands(&self) -> Vec<Box<dyn PluginCommand<Plugin = Self>>> {
         // This is a list of all of the commands you would like Nu to register when your plugin is
         // loaded.