Add browser test suite #39

use-editable has no tests. Add a browser test suite, following react-live's setup (vitest + @vitest/browser-playwright, stories run in real Chromium).

jsdom can't stand in here: it has no contentEditable editing model, so typing, selection, and caret behavior have to be exercised in a real browser.

Coverage to aim for

• Typing: characters land in the right place, and onChange receives the right text and Position
• Caret position: preserved across re-renders, including an unrelated parent re-render mid-keystroke (regression test for fix: don't restore a stale caret position onto content it wasn't computed against #37)
• Undo / redo: Cmd/Ctrl+Z and Shift+Cmd/Ctrl+Z history
• Indentation: opts.indentation Tab/Shift+Tab and backspace behavior
• Edit API: update, insert (including negative offsets), move (offset and { row, column }), getState
• opts.disabled: toggling it on and off
• Line breaks: Enter, <br> handling, trailing newline

Setup

• vitest browser config + Playwright Chromium (see react-live's vitest.browser.mts and stories/)
• npm run test script (kept out of npm run check, like react-live, so the main check stays fast)
• CI job that installs Chromium and runs the suite, including the React 18.0.0 compat leg
• Document in CONTRIBUTING.md

Related: #38 (oxlint). Its hooks-order fix would benefit from these tests landing first.