Improve terminal rendering performance (#33345)

Closes #18263

Improvements:

• **Batch text rendering** - Combine adjacent cells with identical
styling into single text runs to reduce draw calls
• **Throttle hyperlink searches** - Limit hyperlink detection to every
100ms or when mouse moves >5px to reduce CPU usage
• **Pre-allocate collections** - Use `Vec::with_capacity()` for cells,
runs, and regions to minimize reallocations
• **Optimize background regions** - Merge adjacent background rectangles
to reduce number of draw operations
• **Cache selection text** - Only compute terminal selection string when
selection exists

Release Notes:

- Improved terminal rendering performance.

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/gpui/examples/input.rs

@@ -487,7 +487,7 @@ impl Element for TextElement {
         let font_size = style.font_size.to_pixels(window.rem_size());
         let line = window
             .text_system()
-            .shape_line(display_text, font_size, &runs);
+            .shape_line(display_text, font_size, &runs, None);
 
         let cursor_pos = line.x_for_index(cursor);
         let (selection, cursor) = if selected_range.is_empty() {