`merge_group` event

[jkarni]

Issue description:
The merge_group webhook event seems to be missing. It would be great to have it added.

We're happy to contribute this, if it'd be accepted. It seems like the code is not automatically generated in any way, but that the fixtures are taken from the examples in the GitHub docs?

And thanks for the wonderful library!

[kvanbere]

Hi @jkarni,

Something we noticed early on is that the GitHub documentation is lacking and mostly out of date.

The fixtures are a mixture of GitHub documentation samples, but also real dumps from the actual API (which includes some fields that i.e. didn't exist in the documentation or samples at the time, or in some cases undocumented functionality). It would be nice to have more fixtures and better test coverage - we are open to contributions for this.

You are right, the code is not automatically generated, but the Haskell DSL that we use is very succinct so adding more fields or types is very easy.

The build system is very simple -- to get started, I think all you need to do is clone the repo and run stack build or stack test.

Maybe the most difficult thing here is that we support all platforms and all GHC versions since the library's beginning (7 years ago) but there is extensive CI/CD to help with this!