Add initial support for ICU MessageFormat.

[SirStendec]

Proposed changes

This pull request implements a new check for ICU MessageFormat messages. This check:

• Parses ICU MessageFormat messages, performing syntax validation in the process
• Checks that placeholders in source and translations match in name
• Checks that placeholders in source and translations match in type
• Checks that sub-message keys are valid for plural placeholders.
• Checks that sub-message keys in translations are present in the source for select placeholders.
• Supports basic XML tag placeholders, which are used by some libraries for rich content with ICU.
• Can be configured via translation flags, letting users tweak the behavior to fit their strings better.

Specifically, this pull request:

1. Adds the aforementioned check as weblate.checks.icu.ICUMessageFormatCheck
2. Adds a dependency on the ICU MessageFormat parsing class pyicumessageformat which was written for this purpose.
3. Adds unit tests for the new check (sitting at 85% coverage for icu.py)
4. Documents the new check and its associated flags in user/checks.rst and admin/checks.rst

This pull request does not:

• Implement the Live Preview that I have showed screenshots of in issue weblate & icu format #2856.

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

Screenshot of a syntax error:

[codecov]

Codecov Report

Merging #6582 (15851c5) into main (0b5121d) will decrease coverage by 0.00%.
The diff coverage is 90.55%.

❗ Current head 15851c5 differs from pull request most recent head 0a00274. Consider uploading reports for the commit 0a00274 to get more accurate results

@@            Coverage Diff             @@
##             main    #6582      +/-   ##
==========================================
- Coverage   91.41%   91.40%   -0.01%     
==========================================
  Files         624      626       +2     
  Lines       47023    47405     +382     
  Branches     5204     5292      +88     
==========================================
+ Hits        42984    43330     +346     
- Misses       2737     2765      +28     
- Partials     1302     1310       +8     

Impacted Files	Coverage Δ
weblate/checks/models.py	85.91% <ø> (ø)
weblate/trans/models/unit.py	88.55% <0.00%> (-0.26%)	⬇️
weblate/checks/icu.py	86.12% <86.12%> (ø)
weblate/checks/flags.py	86.70% <100.00%> (+0.28%)	⬆️
weblate/checks/tests/test_checks.py	100.00% <100.00%> (ø)
weblate/checks/tests/test_icu_checks.py	100.00% <100.00%> (ø)
weblate/utils/requests.py	93.54% <0.00%> (-6.46%)	⬇️
weblate/checks/base.py	90.84% <0.00%> (+1.30%)	⬆️

[SirStendec]

Sorry, I completely forgot about PrismJS highlighting in the first commit as I was thinking about refactoring the Live Preview feature.

Do you want me to restart this PR on a cleaner branch?

[nijel]

Thanks, looks good! It would be great if you could improve test coverage for the code.

[nijel]

Can you please for more of the format_result branches?

[SirStendec]

Just to be sure I'm not missing something here, did you mean "Can you please test for more [...]"?

Given the earlier comment about increasing test coverage, that's my assumption but I want to be sure.

[nijel]

Yes, that's it. Sorry for missing one word here....

[nijel]

Avoid nesting if statements.

[nijel]

Merged, thanks for your contribution!