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_gstat/src/nu/mod.rs

@@ -1,6 +1,6 @@
 use crate::GStat;
 use nu_plugin::{EngineInterface, EvaluatedCall, Plugin, PluginCommand, SimplePluginCommand};
-use nu_protocol::{Category, LabeledError, PluginSignature, Spanned, SyntaxShape, Value};
+use nu_protocol::{Category, LabeledError, Signature, Spanned, SyntaxShape, Value};
 
 pub struct GStatPlugin;
 
@@ -13,9 +13,16 @@ impl Plugin for GStatPlugin {
 impl SimplePluginCommand for GStat {
     type Plugin = GStatPlugin;
 
-    fn signature(&self) -> PluginSignature {
-        PluginSignature::build("gstat")
-            .usage("Get the git status of a repo")
+    fn name(&self) -> &str {
+        "gstat"
+    }
+
+    fn usage(&self) -> &str {
+        "Get the git status of a repo"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(PluginCommand::name(self))
             .optional("path", SyntaxShape::Filepath, "path to repo")
             .category(Category::Custom("prompt".to_string()))
     }