Feat: extracted method from trait impl is placed in existing impl
**Before**
https://user-images.githubusercontent.com/1759192/183872883-3b0eafd2-d1dc-440e-9e66-38e3372f8b64.mp4
**After**
https://user-images.githubusercontent.com/1759192/183875769-87f34c7d-52f0-4dfc-9766-f591ee738ebb.mp4
Previously, when triggering a method extraction from within an impl trait block, then this would always create a new impl block for
the struct, even if there already is one. Now, if there is already an existing trait-less impl block, then it'll put the extracted method in there.
**Caveats**:
- It currently requires the target impl block to be non-empty. This limitation is because the current architecture takes a `node_to_insert_after` as reference for where to insert the extracted function. An empty impl block doesn't have such a reference node, since it's empty. It seems that supporting this requires a much larger and more complex change.
- This is my first contribution in rust, so apologies for any beginner mistakes.
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