Minor optimization of line number length logic (#26845)

In `layout_excerpt_gutter`, compute max line number length once instead
of for every row

In `max_line_number_width`, use ilog10 instead of converting to floats
and back

Release Notes:

- N/A

crates/multi_buffer/src/multi_buffer.rs

@@ -4096,6 +4096,7 @@ impl MultiBufferSnapshot {
     }
 
     pub fn widest_line_number(&self) -> u32 {
+        // widest_line_number is 0-based, so 1 is added to get the displayed line number.
         self.excerpts.summary().widest_line_number + 1
     }