Enable clippy::useless_conversion (#8724)

This PR enables the
[`clippy::useless_conversion`](https://rust-lang.github.io/rust-clippy/master/index.html#/useless_conversion)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/editor/src/display_map/block_map.rs

@@ -987,7 +987,7 @@ fn offset_for_row(s: &str, target: u32) -> (u32, usize) {
         if row >= target {
             break;
         }
-        offset += line.len() as usize;
+        offset += line.len();
     }
     (row, offset)
 }