Introduce the TypeAbstractions language flag

GHC Proposals #448 "Modern scoped type variables"
and #425 "Invisible binders in type declarations"
introduce a new language extension flag: TypeAbstractions.

Part of the functionality guarded by this flag has already been
implemented, namely type abstractions in constructor patterns, but it
was guarded by a combination of TypeApplications and ScopedTypeVariables
instead of a dedicated language extension flag.

This patch does the following:

* introduces a new language extension flag TypeAbstractions
* requires TypeAbstractions for @a-syntax in constructor patterns
  instead of TypeApplications and ScopedTypeVariables
* creates a User's Guide page for TypeAbstractions and
  moves the "Type Applications in Patterns" section there

To avoid a breaking change, the new flag is implied by
ScopedTypeVariables and is retroactively added to GHC2021.

Metric Decrease:
    MultiLayerModulesTH_OneShot

0 files changed, 0 insertions, 0 deletions