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-protocol/src/engine/engine_state.rs

@@ -530,11 +530,7 @@ impl EngineState {
                     let examples = decl
                         .examples()
                         .into_iter()
-                        .map(|eg| PluginExample {
-                            example: eg.example.into(),
-                            description: eg.description.into(),
-                            result: eg.result,
-                        })
+                        .map(PluginExample::from)
                         .collect();
                     let sig_with_examples = PluginSignature::new(sig, examples);
                     serde_json::to_string_pretty(&sig_with_examples)