Merge #2343

2343: implement assist invert_if r=matklad a=bravomikekilo

fix [issue 2219 invert if condition](https://github.com/rust-analyzer/rust-analyzer/issues/2219)
I put the assist cursor range to `if` of the if expression, because both condition and body will be replaced. Is there any way to replace them without cover the cursor position?
@matklad

Co-authored-by: bravomikekilo <bmk1221@126.com>