4596: Strip leading underscores of argument names in function/method r=matklad a=kuy
Closes#4510
### Goal
When I select a function/method from completions, I get a snippet that doesn't contain leading underscores of argument names.
### Solution
- Option 1: All signatures don't contain underscores
- Option 2: Keep same signature, but inserted snippet doesn't contain underscores
I choose Option 2 because I think that leading underscores is a part of "signature". Users should get correct signatures. On the other hand, trimming underscores is an assist by IDE.
### Other impls.
rls: Complete argument names with underscores (same as actual ra)
IntelliJ Rust: Doesn't complete argument names
VSCode (TypeScript): Doesn't complete argument names
### Working example
Co-authored-by: Yuki Kodama <endflow.net@gmail.com>
59adc7bfb6
