Add zh, zh-Hans-CN, and zh-Hant-TW

[PIG208]

This adds app_zh.arb so that locales like "zh-Hans-CN" can have their
code generated.

This is because flutter gen-l10n requires a base locale to be present.
Only having "zh-Hans-CN" and "zh-Hant-TW" and gives us the following error.

Arb file for a fallback, zh, does not exist, even though 
the following locale(s) exist: [zh_Hans_CN, zh_Hant_TW]. 
When locales specify a script code or country code, a 
base locale (without the script code or country code) should 
exist as the fallback. Please create a {fileName}_zh.arb 
file.

"zh" will not appear on Weblate, though, because we exclude it from
settings. This is intentional since it only makes sense to translate
the more specific locales like "zh-Hans-CN" and "zh-Hant-TW".

CZO discussion:
https://chat.zulip.org/#narrow/channel/58-translation/topic/zh_*.20in.20Weblate/near/2174667

The following are screenshots from my Android device, with different system language preferences set.

zh-Hans-CN	zh-Hant-TW	zh-Hant-HK

(zh-Hant-HK resolves to zh-Hant-TW)

[gnprice]

Cool, thanks.

I just sent #1522 as a pure cherry-pick of the commit that went into v0.0.29. If we merge that first, then that will facilitate comparing v0.0.29 with upcoming versions in main; it should also simplify this change somewhat by taking care of most of the unrelated changes that had accumulated.

[PIG208]

This is also an experiment of how Weblate processes new languages we added. I suppose ar (Arabic) was added to Weblate from our GitHub remote as opposed to the other way around. This time we will see if Weblate successfully ignores zh with a language filter we set up for this.

[gnprice]

I suppose ar (Arabic) was added to Weblate from our GitHub remote as opposed to the other way around.

It was, but as part of the initial setup of Weblate — the file was added last July, and I believe it was a few months later that we first set up Weblate.

More recently, though, we added nb in our repo and let Weblate pick it up from there, and that seemed to work: #mobile-team > weblate translations (#F276) @ 💬 (That one was complicated partly because it involved an alias to connect that to a translation Weblate itself calls nb_NO: #mobile-team > weblate translations (#F276) @ 💬.)

[gnprice]

Thanks! Looks good; merging.