feat: add typed message tags

[bjarn]

Summary

• Add the exported MessageTag interface.
• Reuse MessageTag in single-send, batch-send, message, and webhook types.
• Keep the existing tag() and tags() methods.
• Preserve the current object input format.

Validation

• Enforce the documented name and value formats.
• Reject reserved __lettermint names without regard to case.
• Reject duplicate case-sensitive names.
• Allow 20 total tags, including the legacy single tag.

Documentation

• Add structured tag usage and compatibility notes to the README.

Verification

• npm test -- --runInBand
• npm run build
• npm run lint