feat: Add v8::icu::get_language_tag()

[AnInternetTroll]

closes #797

[CLAassistant]

All committers have signed the CLA.

[ry]

What do you need this for?

[bartlomieju]

What do you need this for?

Needed for denoland/deno#12322

[littledivy]

icu_get_default_locale is doing 3 allocations! Is there a reason? You can just not copy at all:

const char* icu_get_default_locale(size_t* len) {
   // ...
   *len = tag.size();
   return tag.data();
}

let mut len = 0;
let locale: *const u8 = unsafe { icu_get_default_locale(&mut len) };
let bytes = unsafe { std::slice::from_raw_parts(locale, len) };
unsafe { String::from_utf8_unchecked(bytes) }

[bartlomieju]

@littledivy I don't think that would work - the data you get from ICU is stack-allocated and as such can't return it to Rust.

[bartlomieju]

LGTM