Add validation helpers for Tangled DIDs and handles
Add boolean validation functions to check DID and handle formats:
- isValidHandle(): Check if string is valid AT Protocol handle
- isValidTangledDid(): Check if string is valid Tangled DID (did:plc: format)
These helpers return true/false without throwing, useful for parsing
operations. Includes comprehensive test coverage.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>