Merge GHC.Internal.TH.Quote into GHC.Internal.TH.Syntax

Currently GHC.Internal.TH.Quote only contains a single definition, the definition of the Quasiquoter datatype. @sgraf812 suggests that we should merge this module into GHC.Internal.TH.Syntax.

The user-facing exports via Language.Haskell.TH.Syntax and Language.Haskell.TH.Quote would remain the same.

The only slight difficulty here is that currently Language.Haskell.TH.Syntax lacks an explicit exports list. It merely re-exports everything from GHC.Internal.TH.Syntax, so we'd need to add an export list to avoid accidentally re-exporting Quasiquoter from that module and changing the user-facing interface.

See: !12479 (comment 563204)

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