Added feature switch to disable animations in TUI (#6870)

This PR adds support for a new feature flag `tui.animations`. By
default, the TUI uses animations in its welcome screen, "working"
spinners, and "shimmer" effects. This animations can interfere with
screen readers, so it's good to provide a way to disable them.

This change is inspired by [a
PR](https://github.com/openai/codex/pull/4014) contributed by @Orinks.
That PR has faltered a bit, but I think the core idea is sound. This
version incorporates feedback from @aibrahim-oai. In particular:
1. It uses a feature flag (`tui.animations`) rather than the unqualified
CLI key `no-animations`. Feature flags are the preferred way to expose
boolean switches. They are also exposed via CLI command switches.
2. It includes more complete documentation.
3. It disables a few animations that the other PR omitted.

codex-rs/tui/src/chatwidget/tests.rs

@@ -338,6 +338,7 @@ fn make_chatwidget_manual() -> (
         enhanced_keys_supported: false,
         placeholder_text: "Ask Codex to do anything".to_string(),
         disable_paste_burst: false,
+        animations_enabled: cfg.animations,
     });
     let auth_manager = AuthManager::from_auth_for_testing(CodexAuth::from_api_key("test"));
     let widget = ChatWidget {