Rename <Slate> component value prop to initialValue

[e1himself]

Description

The Slate editor is in fact an uncontrolled component, but the public API deceives into thinking it's a controlled input. This is because of the value and onChange is a common React convention for controlled inputs, while uncontrolled inputs use initialValue instead.

Issue
Fixes #4992

Example

Before:

<Slate editor={editor} value={value} />

After:

<Slate editor={editor} initialValue={value} />

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: ec5d95d

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	Minor

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[dylans]

Looks good and makes sense to me. Going to leave it open for a couple of days in case anyone has other feedback.

[atanasov-deyan]

apologies for jumping into an already merged PR, but wouldn't this be a breaking change, as the API is changing? This is not explicitly highlighted anywhere, tbh. wdyt?

[pchr-srf]

What @atanasov-deyan says, just spent more time than I care to admit on trying to find out why stuff wasn't working. Turns out a minor change only visible in https://github.com/ianstormtaylor/slate/blob/main/packages/slate-react/CHANGELOG.md#0950 was the culprit, fixed with a oneliner 🙈