x/text/language: count '_' alongside '-' in ParseAcceptLanguage guard

[tonghuaroot]

The BCP 47 scanner aliases '' to '-' in scanner.init at
internal/language/parse.go lines 104-111, so a payload built
with '' as the separator bypasses the 1000-dash guard at
language/parse.go:169 (the CVE-2022-32149 fix landed in CL
442235) and re-triggers the O(N^2) gobble path on
attacker-controlled Accept-Language input. Count both
separators in the guard.

Reproduction summary:

100k tokens with '_' separator: 1.35 s parse time
Same 100k tokens with '-': 49.9 us (guard fires)
Amplification: ~27,000x

See golang/go#79684 for the full reproducer with go.mod,
main.go, and measurement output.

Test:

TestParseAcceptLanguageUnderscoreGuard in language/parse_test.go
asserts errTagListTooLarge is returned when 1001 '_' separators
are present.

Filed per the upstream invitation from the Go Security Officers:
the original security@golang.org report was determined to be not
a security issue, with an invitation to file as an upstream
change.

Fixes golang/go#79684

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Patch Set 1:

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Patch Set 2: Code-Review+2

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