Use memoization to avoid unnecessary textContent updates in TextString component

[clauderic]

Description
This PR changes the approach used in the <TextString> component to synchronize the DOM and the Virtual DOM.

Previously, the initial render of the <TextString> component would render the initial text, but the second render would remove the text from the DOM, before quickly being re-committed to the DOM in the layout effect before the browser had time to paint.

This PR takes a different approach that avoids ever having to render an empty textContent on the second render. Instead, we render a memoized component in <TextString> (<MemoizedText>), that receives props that are referentially stable across re-renders, such that React never updates the DOM across re-renders.

This PR provides a significant performance improvement over the current implementation, as previously the first update to an element would cause all its text leaves to re-render and wastefully update their textContent even if it had not changed.

Issue
I couldn't find any related tickets.

Example

Before

Screen.Recording.2023-02-22.at.5.41.31.PM.mov

After

Screen.Recording.2023-02-22.at.5.40.35.PM.mov

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: b6dd630

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]

This looks promising. Going to try this out in our app in collaborative editing mode and see if it introduces any regressions.