Switch to ICU MessageFormat

[askvortsov1]

Foundation for flarum/issue-archive#385
Closes #2745
Closes #2062 as moot

Changes proposed in this pull request:
Done:

• Support +intl-icu files in locales and language packs for backend translator
• Add integration tests for locale extender (can't for language pack extender, as that can only run in an extension)

TODO:

• Wait for Add terser minification step ultraq/icu-message-formatter#10, Support offset arg in pluralTypeHandler ultraq/icu-message-formatter#9 to be merged (if they will be, but feels likely)
• If Add rich formatting method ultraq/icu-message-formatter#11 isn't merged, we'll need to include it ourselves. I would prefer to put it in a separate package for now, as I'm not comfortable including it without the tests. Parsers tend to be ugly 😆

Reviewers should focus on:

• Should +intl-icu be the default for ALL files? This might have slight performance ramifications, but it would also be more consistent (see expected results of custom_translation_exists_if_added vs custom_translation_does_not_exist_by_default in the backend tests). This would be a further breaking change though. But it would be more consistent with what's done on the frontend. Currently in the PR.
• We still need a translation args pre-processing system, but I think that should go in a second PR.

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

[davwheat]

I'm for the first option too. Smaller bundle! 👍

[tankerkiller125]

Maybe we should include +intl-icu as a prefix by default? That way language packs don't have too? But I'm not 100% certain on that one.

[askvortsov1]

Maybe we should include +intl-icu as a prefix by default? That way language packs don't have too? But I'm not 100% certain on that one.

This might hurt performance, as MessageFormat is more complex than non-MessageFormat. However, consistency would be a benefit: we would no longer need to include curly braces in variable keys. Plus, we're using MessageFormat code in the frontend anyway.

[askvortsov1]

I'm for the first option too. Smaller bundle!

I got it working for 95% of what we need via ultraq/icu-message-formatter#7, other needed changes are outlined in the same issue. If that isn't merged, we can just include the code from the PR in our repo; this library is very extensible!

[askvortsov1]

After some more thought, why don't we ise intl+icu for everything, and transform backend keys to remove wrapping curly braces if present as a BC layer?

[askvortsov1]

This is ready for review, but we can't merge until the items in TODO are implemented over in https://github.com/ultraq/icu-message-formatter

[askvortsov1]

No longer WIP, this is now ready for review.

[luceos]

This looks awesome, but please involve our translators if this hasn't happened yet (a discussion under the i18n - https://discuss.flarum.org/t/i18n - tag would be smart. At least they need to know in advance because this will be NBC for all translations.

[askvortsov1]

https://discuss.flarum.org/d/27030-upcoming-translation-changes

[tankerkiller125]

Looks good