'DynFlag'-free version of 'mkParserFlags'

Obtaining a `DynFlags` is difficult, making using the lexer/parser
for pure parsing/lexing unreasonably difficult, even with
`mkPStatePure`.
This is despite the fact that we only really need

  * language extension flags
  * warning flags
  * a handful of boolean options

The new `mkParserFlags'` function makes is easier to directly construct
a `ParserFlags`. Furthermore, since `pExtsBitmap` is just a footgun,
I've gone ahead and made `ParserFlags` an abstract type.

Reviewers: bgamari, alanz, sjakobi

Reviewed By: bgamari, sjakobi

Subscribers: mpickering, sjakobi, rwbarton, carter

GHC Trac Issues: #11301

Differential Revision: https://phabricator.haskell.org/D5269

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