Make get_full_help take &dyn Command (#12903)

# Description
Changes `get_full_help` to take a `&dyn Command` instead of multiple
arguments (`&Signature`, `&Examples` `is_parser_keyword`). All of these
arguments can be gathered from a `Command`, so there is no need to pass
the pieces to `get_full_help`.

This PR also fixes an issue where the search terms are not shown if
`--help` is used on a command.

crates/nu-plugin-engine/src/context.rs

@@ -139,14 +139,10 @@ impl<'a> PluginExecutionContext for PluginExecutionCommandContext<'a> {
     fn get_help(&self) -> Result<Spanned<String>, ShellError> {
         let decl = self.engine_state.get_decl(self.call.decl_id);
 
-        Ok(get_full_help(
-            &decl.signature(),
-            &decl.examples(),
-            &self.engine_state,
-            &mut self.stack.clone(),
-            false,
+        Ok(
+            get_full_help(decl, &self.engine_state, &mut self.stack.clone())
+                .into_spanned(self.call.head),
         )
-        .into_spanned(self.call.head))
     }
 
     fn get_span_contents(&self, span: Span) -> Result<Spanned<Vec<u8>>, ShellError> {