language: limit TextUnmarshaler tag complexity

[carrerasdarren-cell]

The BCP 47 tag parser has inherent quadratic time complexity.
ParseAcceptLanguage bounds that work, but Tag.UnmarshalText still passes
untrusted JSON, XML, and direct text input to the parser without a
complexity limit.

Reject inputs with more than 1000 total dash and underscore separators
before parsing. Count both spellings because the scanner accepts each as
a tag separator. Add regression coverage for both forms.

A 320 kB reproduction with 32,000 invalid subtags dropped from 71-77 ms
to under 1 ms. The complete x/text test suite, focused race tests, and
go vet pass.

Fixes golang/go#80160.

[gopherbot]

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