Repo housekeeping + support for React 18/19

[srubin]

• Use pnpm instead of yarn
• Use rollup/vite instead of gulp/webpack
• Upgrade typescript
• Replace examples directory with an examples target that can be developed alongside the main draft.js code
• Add support for React 18/19 (afaik, this only requires switching out the old flush function for the new one)
• DOM tests needed to be updated to work with the latest react/react-dom.

Nearly all of this was done by Claude Code, save for the actual change that we needed to make for proper React 18/19 support (the use of flushSync). I've wanted to update the repo to a more modern toolchain for a while, but was avoiding it while we were still occasionally pulling upstream fixes. But now that the official repo is archived, we can go wild 🥳

The two risks here are (1) the tests are no longer testing the right thing, and (2) there's an issue with the new build process that's breaking something. We'll check these empirically by checking the new package against our large suite of text editor UI tests we have in our private repo, and with manual smoke testing.

• Confirmed that the package works as expected locally in our private repo when installing it via file:

[srubin]

The previous unstable method that was being used here was actually already removed before the version of React that we're using, so I'm not going to bother with the fallback for react < 18. facebook/react#26397