tab: Add setting to hide the close button entirely (#23880)

Closes #23744

Release Notes:

- Changed the `always_show_close_button` key to `show_close_button` and
introduced a new `hidden` value, that allows never displaying the close
button.

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Co-authored-by: Peter Tripp <peter@zed.dev>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: smit <0xtimsb@gmail.com>