Introduce IANA-BCP47 to validate the IETF language code

[Masterain98]

Hello again,

This PR introduces a more robust solution for validating user input of IETF language codes using the iana-bcp47 library. Compared to the older bcp47 library, this new implementation validates each subtag against the IANA Language Subtag Registry, rather than relying on a predefined whitelist.

Context

highfestiva/bcp47.py#7

In my experience with an older version of pymkv, valid language codes such as zh-Hans-CN would cause a ValueError during validation. These codes are valid according to BCP47 standards but were not included in the whitelist used by bcp47. To address this issue, I decided to implement a validator that strictly adheres to IANA's rules, which this PR introduces.

What's Changed

Previously, the is_bcp47 function was removed, and user input was directly set as the language metadata. While I have not tested whether incorrect language codes can cause issues when MKV files are loaded in other software, I believe a simple validation mechanism should still be included.

With the proposed logic, if the user-entered language code fails validation:

• A warning message is displayed with specific details about the error.
• The language of the MKVTrack is set to None if the code is invalid.

This ensures that language metadata conforms to IANA standards while providing helpful feedback to users.

source of iana-bcp47: https://github.com/Masterain98/iana-bcp47

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.98%. Comparing base (b3e1883) to head (ea22f79).

Files with missing lines	Patch %	Lines
pymkv/MKVTrack.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   66.87%   66.98%   +0.10%     
==========================================
  Files          10       10              
  Lines         948      954       +6     
==========================================
+ Hits          634      639       +5     
- Misses        314      315       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[GitBib]

The iana-bcp47 library doesn’t appear fully vetted yet, and it’s lacking solid tests. I need some time to review it, and after that, I’ll decide whether to include iana-bcp47 as a dependency.