Add ReactEditor.isComposing(editor)

[BitPhinix]

Description

Moves the Editable isComposing state into the isComposing WeakMap used by the AndroidEditable and exposes the current state via ReactEditor.isComposing(editor).

It's quite common to handle some events differently inside a user-defined handler based on the isComposing state (e.g. you don't want to trigger markdown shortcuts etc. while the user is composing as that often leads to undesirable behavior).

Currently, you have to keep track of the isComposing state on your own using your own, which I don't think is desirable.

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: 68f28c5

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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