Add locale support to theme provider and use in date-text column

[m-akinc]

Pull Request

🤨 Rationale

Fixes #1293
Part of #1014
Adding design token ("lang") for locale and honoring that token's value when formatting dates in the date-text column.

👩‍💻 Implementation

• introduced "lang" design token with dynamic default that uses the page's lang property, if valid, otherwise uses "en-US"
• new DocumentElementLang singleton watches page element's lang property
• added lang property to theme provider which sets design token value
• using design token value when creating formatter used by date-text column
• added new validation flag to theme provider that is set when the locale is malformed
• setting 'en-US' locale for all date-text unit tests (which they were already assuming)
• added unit tests

🧪 Testing

Unit tests pass (even when system locale is something other than 'en').

✅ Checklist

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