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/date/date_.rs

@@ -42,26 +42,6 @@ impl Command for Date {
         call: &Call,
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
-        date(engine_state, stack, call)
+        Ok(Value::string(get_full_help(self, engine_state, stack), call.head).into_pipeline_data())
     }
 }
-
-fn date(
-    engine_state: &EngineState,
-    stack: &mut Stack,
-    call: &Call,
-) -> Result<PipelineData, ShellError> {
-    let head = call.head;
-
-    Ok(Value::string(
-        get_full_help(
-            &Date.signature(),
-            &Date.examples(),
-            engine_state,
-            stack,
-            false,
-        ),
-        head,
-    )
-    .into_pipeline_data())
-}