10413: fix: Fix format highlighting with `concat!` and `unreachable!` r=jonas-schievink a=jonas-schievink Last item in https://github.com/rust-analyzer/rust-analyzer/issues/10394, closes https://github.com/rust-analyzer/rust-analyzer/issues/10394 Also documents why/how `is_format_string` even works, since I found that nontrivial. bors r+ Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>