Fix panic on raw string assist
Fix panic on raw string assist

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.

rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer