Show which roots are being scanned in progress messages
This changes the `Roots Scanned` message to include the directory being scanned.
Before: `Roots Scanned 206/210 (98%)`
After: `Roots Scanned 206/210: .direnv (98%)`
This makes it a lot easier to tell that `rust-analyzer` isn't crashed, it's just trying to scan a huge directory.
See: #12613