x/text/language: language Matcher does not properly support zh_HK (Hong Kong)

[pkpfr]

What version of Go are you using (go version)?

$ go version
1.13

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

Reproduces on all processors and architectures

What did you do?

Tried to match the following language header from Google Chrome:

zh-HK,zh;q=0.9,zh;q=0.8,en-GB;q=0.7,en;q=0.6,ja;q=0.5,th;q=0.4,en-US;q=0.3

https://play.golang.org/p/hQ_FUehpgoZ

What did you expect to see?

Language matcher returns the Hong Kong region and Traditional Chinese character set

What did you see instead?

Language matcher is unaware of the Hong Kong region and falls back to the next language (Chinese or Taiwanese). Does not return traditional character set unless zh-Hant or zh-TW is explicitly set in the language header.

[toothrot]

/cc @mpvl who can weigh in on this.