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/pager_overlay.rs

@@ -722,6 +722,7 @@ mod tests {
             vec![ParsedCommand::Unknown { cmd: "ls".into() }],
             ExecCommandSource::Agent,
             None,
+            true,
         );
         exec_cell.complete_call(
             "exec-1",