IsWellFormedLanguageTag doesn't match well-formedness from UTS 35 for duplicate ukey or tkey values

[anba]

Well-formedness constraints include:

There cannot be more than one ukey or tkey. For example, en-u-ca-buddhist-ca-islamic is ill-formed.

ECMA-402 doesn't have this restriction, i.e. IsWellFormedLanguageTag("en-u-ca-buddhist-ca-islamic") returns true (and can't be changed to return false).

See also #1008.

[hsivonen]

@zbraniecki What are the implications for icu_locale_core and the applicability of ICU4X as an ECMA-402 back end here?