Add soft break as a separate overridable editor method

[bryanph]

Description
A different behavior for inserting a soft break with shift+enter is quite common in rich text editors. Right now you have to do this in onKeyDown which is not so nice. This adds a separate insertSoftBreak method on the editor instance that gets called when a soft break is inserted. I made sure to maintain the current default behavior for backwards compatibility (it just splits the block). But at least you can easily overwrite it now.

If you rely on overwriting editor.insertBreak for extra behavior for soft breaks this might be a breaking change for you and you should overwrite editor.insertSoftBreak instead.

Issue
Fixes: (link to issue)

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: 973ea90

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

This PR includes changesets to release 2 packages

Name	Type
slate	Minor
slate-react	Minor

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[dylans]

overall I like it, but I'm going to leave it open for a few days for others to give feedback.