Implement parsing gen fn

[narpfel]

Generator functions were implemented in rust-lang/rust#118457, tracking issue rust-lang/rust#117078.

[wetneb]

Hi @narpfel , I have noticed that PRs in this repository are infrequently reviewed, so I have integrated your PR in a fork of this grammar. If you use this grammar in Rust, you can benefit from the improvements in this PR by switching to the tree-sitter-rust-orchard crate on crates.io. Let me know if you need the grammar published in another package repository. I would very much welcome more contributions like this one in the fork, to which I would also be happy to give you write access.

Dear maintainers,
Thank you for your work. It is completely understandable that you have higher priorities in life than integrating external contributions in this repository. If you still wish those contributions to get reviewed, I encourage you to give maintainer rights to more people. The list of PRs and the commit log should help you identify trustworthy contributors who might be interested in helping out. Onboarding new contributors might also be easier if you adopt a governance model for this project. If there is any way I can help, let me know.

[narpfel]

Rebased onto current master.