Draft: Add `instance Read Extension`

Thank you for your contribution to GHC!

Please take a few moments to verify that your commits fulfill the following:

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• The type Extension has a Show instance but no corresponding Read instance. This complicates any code that attempts to parse Extension for the GHC API. As an example, I am currently patching retrie to accept language extensions on the command line. This change requires a parser for Extension. The easiest thing to do is throw an orphan instance for Read Extension, which I've done. So, in due diligence, I'm making this MR to either get it merged or get pointed to a nicer solution.

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.