Add missing Vietnamese diacritics

[huy-vuong]

Context

• Source: https://en.wiktionary.org/wiki/%C3%94#See_also_3
  • This section shows a mapping of characters in the alphabet (some of which have diacritics) to characters with additional diacritics. This reflects all possible diacritic combinations in Vietnamese.
  • The diff in this PR adds support for any mappings not already covered in the existing code.
    • This does not include adding any mappings where no diacritics are present, such as the mappings for N.

Testing

• Tests in test.js updated to include the new mappings and pass.

[tyxla]

Thanks @huy-vuong, this looks great 👍

🙌

[tyxla]

@huy-vuong just noticed that this PR was opened from your fork's master branch.

Would you mind opening it from a new branch on your fork and reopening a PR?

Thanks in advance!

[tyxla]

As mentioned above, this needs to be on a different branch than master in order to land.

Will also need to be rebased to address merge conflicts.