Export parser

It can be useful to parse BCP47 tags in the context of a larger parser,
such as parsing the HTTP Accept-Language Header, which is formatted as

    {language}[;q={weight}][,...]

Doing this on top of fromText is possible, but a bit annoying.

NOTE: I attempted to move this to a Data.BCP47.Parser, but the current
top-level module contains too many lower-level data definitions for that
to be possible without an import-cycle. To address that, we could move
most of Data.BCP47 to .Internal modules themselves, and leave Data.BCP47
as a re-export, higher-level interface for the most common usage. This
would be a Good Thing, IMO, but has been deferred for now.

bcp47/library/Data/BCP47.hs

@@ -26,6 +26,7 @@ module Data.BCP47
   , mkLanguage
   , mkLocalized
   , fromText
+  , parser
   -- * Serialization
   , toText
   -- * Subtags