Refactor metadata PO files generation

[iangmaia]

Fixes AINFRA-1724
See discussion on Slack: p1767962906955309-slack-CC7L49W13
See test PR: woocommerce/woocommerce-ios#16502

Summary

The main goal of this PR is to fix the issue when generating the .po / .pot files where new entries aren't present in the output or removed entries are removed. To do that, I've consolidated the metadata PO file generation into a new PoFileGenerator class using gettext. The ios_update_metadata_source and an_update_metadata_source actions are now deprecated and delegate to gp_update_metadata_source.

What's Changed

New Features:

• Translator comments: source_files now accepts { path: 'file.txt', comment: 'translators: ...' } format
• commit_changes option: Auto-commit support on gp_update_metadata_source

Output Changes:

• Entries sorted alphabetically by msgctxt for deterministic output
• Translator comments (#. lines) must now be explicitly provided—they're no longer preserved from existing PO files
• PO-Revision-Date is now updated with the current date time

Breaking Changes

1. Entry order will change in generated PO files
2. Translator comments will be lost unless migrated to the new format:

# Before
source_files: { app_store_subtitle: 'path/to/subtitle.txt' }

# After (to preserve comments)
source_files: {
  app_store_subtitle: {
    path: 'path/to/subtitle.txt',
    comment: 'translators: Limit to 30 characters!'
  }
}

Migration

When upgrading, callers need to copy comments from .po/.pot files to the gp_update_metadata_source call site in the Fastfile to make sure the comments remain in the generated file. See MIGRATION.md for details.

Deprecations

Old Action	Replacement
ios_update_metadata_source	gp_update_metadata_source with commit_changes: true
an_update_metadata_source	gp_update_metadata_source

Deprecated actions still work (API unchanged) but the output will be different (the comments and header will be removed and the keys will be sorted alphabetically, mainly).

[dangermattic]

	1 Warning
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