11218: fix: Correct has_ref detection, avoiding duplicate &mut insertion on parameter completion r=Veykril a=weirane
The original code fails to detect there's a ref in scenarios such as `&mut s` and `& s` because `WHITESPACE` and `IDENT` got reversed.
Closes#11199.
Co-authored-by: Wang Ruochen <wrc@ruo-chen.wang>