feat: add bcp 47 language tag validator

[joshuaslate]

Fixes Or Enhances

This adds a new validator: language_tag, which uses x/language to parse a string to validate that it's a BCP 47 tag.

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• Tests exist or have been written that cover this particular change.

@go-playground/admins

[coveralls]

Coverage increased (+0.02%) to 76.094% when pulling aaeabe8 on joshuaslate:feat/lang_tag_validator into f6584a4 on go-playground:master.