Feature: add Vietnamese dictionary

[NDKhanh96]

This PR adds Vietnamese spell checking support using Hunspell
dictionaries sourced from cspell-dicts.

• Adds Vietnamese (vi) dictionary
• Uses official vi_vn Hunspell files

[Copilot]

Pull request overview

This PR adds Vietnamese spell checking support to Codebook by integrating Hunspell dictionaries from the cspell-dicts repository. The changes enable users to spell-check Vietnamese text using the "vi_vn" dictionary option.

Changes:

• Added Vietnamese (vi_vn) dictionary configuration with Hunspell dictionary URLs
• Updated README documentation to list Vietnamese as an available dictionary
• Added Vietnamese example file for testing the spell checker

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
crates/codebook/src/dictionaries/repo.rs	Adds Vietnamese dictionary entry with URLs to cspell-dicts Hunspell files; also updates Polish dictionary URLs
README.md	Documents Vietnamese dictionary availability in configuration options
examples/vi_vn.md	Provides Vietnamese text example for spell checker testing

[blopker]

Hey! Thanks for this. I think the merge commit reverted the url from the pl dictionary though. I can merge once that's fixed!

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[blopker]

Thank you! This will go out with the next release.