Allow passing custom editor creator to slate-hyperscript createEditor

[bryanph]

Description
Just like in slate's core, I use slate-hyperscript to perform tests for my particular editor. This change makes it easier to create your own testing setup by allowing you to pass your own slate Editor instance to slate-hyperscript's createEditor. This way you can simply reuse the setup code that's in there without needing to duplicate that code in your own setup.

Issue
Fixes: (link to issue)

Example
createHyperscript would then look like this

const jsx = createHyperscript({
  creators: {
    editor: createEditor(aFunctionThatReturnsAnEditorObject)
  },
  elements: {
    block: { type: 'block' },
    inline: { type: 'inline' }
  }
})

createEditor then takes care of setting the anchor and focus properly given the JSX.

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: 9db4e15

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

This PR includes changesets to release 1 package

Name	Type
slate-hyperscript	Patch

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[dylans]

Please add a changeset when you get a chance, thanks!

[bryanph]

@dylans done!