Rich Text Editor | Add label provider in nimble-rich-text-editor

[aagash-ni]

Pull Request

🤨 Rationale

Issue Link : #1288

Currently, rich-text-editor has title and text content for formatting buttons (Bold, Italics, Bulleted List, Numbered List) and this needs to be localized from the client side.

👩‍💻 Implementation

Reference PR for implementation: #1328

This PR creates label DesignTokens in nimble-rich-text-editor

Note: Angular Integration will be taken up once after this PR completion.

🧪 Testing

• Verified that each label appears in the expected spots in the DOM for the affected components. Note that the current labels are mostly not user-visible. Future format button additions will introduce new visible label tokens.
• Added autotests
  • For label providers: verify token naming, and that label provider attribute updates result in token updates
• Updated Storybook
  • Rich Text Editor has a new "Localizable labels" row in their Storybook controls/API section
  • Under Tokens/Label Providers is a new Rich Text Editor Label Provider.

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.

[jattasNI]

I added @msmithNI as an optional reviewer since he created the label provider infrastructure. But you don't need to wait on his approval. Now that Milan's back in office, you should wait for his review though.