7572: Add `find_or_create_impl_block` to assist utils r=matklad a=yoshuawuyts
This is another continuation of https://github.com/rust-analyzer/rust-analyzer/pull/7562, introducing a small util to either find an `impl` block, or create a new one if none exists. I copied this code from the `generate_new` assist into https://github.com/rust-analyzer/rust-analyzer/pull/7562, and this unifies both into a helper.
It doesn't feel super polished in its current state, but my hope is that this is enough of a starting point that it can be expanded on later. For example something that would be useful would be a flag which either returns the index of the start of the block, or the end of the block.
Anyway, I hope this is useful. Thanks!
Co-authored-by: Yoshua Wuyts <yoshuawuyts@gmail.com>
