10152: feat: Add completion for raw identifiers r=matklad a=nabakin
![rust_analyzer_pr](https://user-images.githubusercontent.com/894305/132110362-c21b713d-acaf-4a6d-9749-ff812172cbce.gif)
Adds support for valid Rust completion of raw identifiers.
Previously, code completion of fields made via raw identifiers would not re-insert those raw identifiers, resulting in invalid Rust code. Now, code completion of fields made via raw identifiers do re-insert those raw identifiers, resulting in valid Rust code.
The same is true for all code completion instances for fields and compatible Rust identifiers.
Co-authored-by: Blake Wyatt <894305+nabakin@users.noreply.github.com>