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-command/src/formats/to/command.rs

@@ -29,16 +29,6 @@ impl Command for To {
         call: &Call,
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
-        Ok(Value::string(
-            get_full_help(
-                &To.signature(),
-                &To.examples(),
-                engine_state,
-                stack,
-                self.is_keyword(),
-            ),
-            call.head,
-        )
-        .into_pipeline_data())
+        Ok(Value::string(get_full_help(self, engine_state, stack), call.head).into_pipeline_data())
     }
 }