Hide Inline Assist button if assistant.button is disabled (#7932)

This PR adds check for `assistant.button` setting in quick bar, to hide
it when the setting is set to false. It seems that the setting can be a
separate one, I would be happy to add it if needed.

Release Notes:

- Improved `assistant.button` setting so that `Inline Assist` button in
editor quick bar is also hidden
([#4500](https://github.com/zed-industries/zed/issues/4500)).

crates/assistant/src/assistant.rs

@@ -1,5 +1,5 @@
 pub mod assistant_panel;
-mod assistant_settings;
+pub mod assistant_settings;
 mod codegen;
 mod prompts;
 mod streaming_diff;