feat: add space after specific keywords in completion
fix#17428.
When completing some specific keywords, it would be convenient if r-a could automatically add a space afterwards.
This PR implements this feature for the following keywords:
- Visibility: `pub`, `pub(crate)`, `pub(super)`, `pub(in xxx)`
- Pattern: `ref` / `mut`
- Others: `unsafe` / `for` / `where`