Add Thai (th) localization

[silkyland]

This PR adds Thai (th) localization to the flutter-quill project. It includes a complete Thai translation for all existing UI elements and messages, ensuring proper handling of Thai characters and text direction. The addition of Thai language support will make flutter-quill more accessible to Thai-speaking users and developers, expanding the project's global reach.

Key changes:

1. Added new localization file quill_th.arb for Thai language.
2. Translated all existing UI strings to Thai.
3. Ensured compatibility with Thai text rendering and input methods.
4. Tested to confirm correct display of Thai text and appropriate UI adaptation.

Related Issues

• Related #[issue number for localization enhancements, if any]

Type of Change

• ✨ New feature: Adds new functionality without breaking existing features.

Suggestions

• It may be beneficial to have a native Thai speaker review the translations for naturalness and context-appropriateness.