Rename Regex to GrammarRegex to avoid confusion

Cabal/Cabal.cabal

Cabal/Distribution/FieldGrammar/Described.hs

Cabal/Distribution/ModuleName.hs

Cabal/Distribution/Parsec/Newtypes.hs

Cabal/Distribution/Utils/GrammarRegex.hs

Cabal/doc/buildinfo-fields-reference.rst

Cabal/tests/UnitTests/Distribution/Described.hs

buildinfo-reference-generator/src/Main.hs

buildinfo-reference-generator/template.zinza

cabal-install/Distribution/Client/Types/RepoName.hs

...-install/tests/UnitTests/Distribution/Client/Described.hs