Bumps peer dep for React to v19. It ... seems to work?
Caveats:
- Still pinned to CodeMirror 5, even though CodeMirror 6 is out now. I haven't had time (nor any real motivation) to investigate what changes are required there. PRs welcome.
- The entire build process is is still CommonJS-based, and a lot of the new tooling (especially the Vite-based stuff) works better with or expects ESM, so you'll need to work around that.
Fixes #319