Block Editor: Add README for Typewriter component

[Hassams]

What?

Adds a README.md for the Typewriter component in block-editor, which previously had none.

Why?

The Typewriter component and its useTypewriter hook had no documentation describing their purpose, usage, or the fact that the hook is exposed outside the package as the private __unstableUseTypewriter API consumed by the editor package's VisualEditor. Most other block-editor components have a README; this one was missing.

How?

Added a README following the conventions used by other component READMEs in this package: a short description, a usage example for the Typewriter wrapper component, and a usage example for the useTypewriter hook.

Testing Instructions

Docs-only change, no functional code touched.

• Read through packages/block-editor/src/components/typewriter/README.md and confirm it accurately describes the component's behavior in packages/block-editor/src/components/typewriter/index.js.

[github-actions]

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[github-actions]

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