fix: cursor column after accepting completion with additionalTextEdits

[ohogb]

pub fn funcname(a: &mut i32) { ... }

let mut a = 0;
funcname(|)
// accept completion `&mut a`

// before:
funcname(&|mut a)

// after:
funcname(&mut a|)

Please Note:

• This doesn't fix cursor row
• I have only tested this briefly with rust-analyzer, so please do more testing before merging (might very well break other stuff)

Related f0ab5e5, ab75cad
Somewhat related #223

[Saghen]

This was super tricky to get working well, but I think I have something that works for both auto imports and the &mut a case. Lmk if you run into any issues with it. Thanks for the patch! Made it easier to figure this out