feat: add support for wxl and apple xliff formats

[nijel]

These were rencently introduced to the translate-toolkit.

[codecov]

Codecov Report

❌ Patch coverage is 87.91209% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.02%. Comparing base (f3d0988) to head (c831de5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
weblate/formats/convert.py	82.05%	4 Missing and 3 partials ⚠️
weblate/formats/ttkit.py	81.81%	3 Missing and 1 partial ⚠️

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[Copilot]

Pull request overview

Adds support for two new translate-toolkit-backed formats in Weblate: WiX Toolset WXL (as a convert-based monolingual format) and XLIFF 1.2 with Apple extensions (as a TTKit format), along with tests and documentation updates.

Changes:

• Introduce WXLFormat (convert-based) and integrate WXL conversion/saving logic.
• Add AppleXliffFormat with zero-based CLDR plural handling shared via a new mixin.
• Add/adjust tests and documentation pages + changelog entry.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
weblate/trans/tests/data/cs-cz.wxl	Adds a WXL fixture used by conversion tests and docs.
weblate/formats/ttkit.py	Adds Apple XLIFF format + shared plural mixin; refactors Stringsdict plural handling to reuse it.
weblate/formats/tests/test_formats.py	Registers a basic test class for the new Apple XLIFF format.
weblate/formats/tests/test_convert.py	Adds conversion round-trip tests for WXL; tweaks convert test diff output.
weblate/formats/models.py	Registers Apple XLIFF format in the formats list.
weblate/formats/convert.py	Adds WXL parser/serializer integration and generalizes convert_to_po to read from configurable unit attributes.
weblate/formats/base.py	Adds get_language_bcp_long_lower() to support lowercase long BCP language codes.
docs/formats/xliff.rst	Documents the new Apple XLIFF variant.
docs/formats/wxl.rst	New documentation page for the WXL format.
docs/changes.rst	Changelog entry mentioning the newly supported formats.

[Copilot]

The example/test WXL file is named cs-cz.wxl, but the root Culture attribute is set to de-de. This is confusing for both documentation and tests; consider aligning Culture with the filename/language being exercised (e.g. cs-cz).

Suggested change

	<WixLocalization xmlns="http://wixtoolset.org/schemas/v4/wxl" Culture="de-de" Codepage="65001">
	<WixLocalization xmlns="http://wixtoolset.org/schemas/v4/wxl" Culture="cs-cz" Codepage="65001">