Let Tab leave the editor outside lists

MDXEditor's lists plugin installs Lexical's tab indentation for every block,
so Tab never left the editor: headings and paragraphs gained an indent MDX
cannot express, and text gained literal tab characters. A higher-priority
KEY_TAB_COMMAND handler now nests and un-nests list items from anywhere in
their text, types a tab in code blocks, leaves table cells to Lexical's table
handler, and otherwise lets the browser move focus.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>