Update to React 19

[s-sbo]

Description Updates Slate's React version to 19.0.0.

Context

• useRef now requires an argument
  • All useRef usage now has an initial argument, with relevant hooks updated to explicitly handle passed refs which may be falsy.
  • Removed depreciated MutableRefObject
• React 19 supports ref as a prop and forwardRef will be depreciated and removed in future.
  • All forwardRef usage has been refactored.
  • (I thought about leaving this for a later refactor as I don't know much about how version bumps are managed on packages like this but in the end the new API reduces so much boilerplate it seemed worth it.)
• Minor housekeeping
  • Updated types, removed unused imports, fixed 404'ing images in images example, minor updates to some other examples

Checks

• [x] The new code matches the existing patterns and styles.
• [] The tests pass with yarn test.
• [x] The linter passes with yarn lint. (Fix errors with yarn fix.)
• [ ] The relevant examples still work. (Run examples with yarn start.)
• [ ] You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: e170f95a4c0b25ed0d2ff9904682c4c0946cf0d7

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[DCzajkowski]

Hi, thanks for the effort! I think the only way to get this PR forward is by splitting it up into separate PRs:

1. Upgrade to React 19 and fix problems,
2. Simplification, updated types, removed unused imports, fixed 404'ing images in images example, minor updates to some other examples, upgraded Yarn etc.

I don't think this PR has a chance to be merged in current state, because it changes too much.