2099: Fix panic on raw string assist r=matklad a=aee11
Strings that do not contain two quotation marks would cause a slice indexing panic because `find_usual_string_range` would return a range that only contained a single quotation mark.
Panic example:
```
fn main() {
let s = "<|>
}
```
I noticed a lot of panics from the `make_raw_string` assist while working on another issue today.
Co-authored-by: Alexander Elís Ebenesersson <alex2789@gmail.com>
Strings that do not contain two quotation marks
would cause a slice indexing panic because code
was assuming `find_usual_string_range` would return
a string with two quotes, but it would incorrectly
also return text ranges containing only a single quote.
