Add TypeAbstractions extension

[ffaf1]

Towards #9496.

If needed, the correct cabal check behaviour can be backported to 3.10 without adding | TypeAbstractions constructor (a breaking change).

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!) We do not write tests on adding extensions.

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QA Notes

1. mkdir prova && cd prova
2. cabal init -nm
3. add a default-extensions: TypeAbstractions in the executable stanza
4. cabal check should not complain with unknown-extension (compare with 3.10 where it does)

[Mikolaj]

Thank you! As noted by Andrea, a backport would need to be hacked specially, without adding the constructor that would violate PvP. (And, IMHO, having this in 3.10 is worth the extra effort, if we can afford it.)