fix: remove memory leaks in Text and Editable

[edhager]

Description
It was discovered that slate-react was leaving behind detached spans and divs as the edited text was being updated causing memory leaks.

The cause of the leaks were in the weak maps used in packages/slate-react/src/components/editable.tsx and packages/slate-react/src/components/text.tsx. DOM nodes stored in the maps were detached but the keys still had strong references.

This PR adds callback refs to remove DOM nodes from the maps when the refs are set to null.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• 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]

🦋 Changeset detected

Latest commit: f874c5d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate-react	Patch

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[dylans]

Looks good, tests pass, etc.

We'll need to add a changeset for this when you get a chance (see instructions).