Introduce InteractiveText (#3397)

This new element will let us react to click events on arbitrary ranges
of some rendered text, e.g.:

```rs
InteractiveText::new(
    "element-id",
    StyledText::new("Hello world, how is it going?").with_runs(vec![
        cx.text_style().to_run(6),
        TextRun {
            background_color: Some(green()),
            ..cx.text_style().to_run(5)
        },
        cx.text_style().to_run(18),
    ]),
)
.on_click(vec![2..4, 1..3, 7..9], |range_ix, cx| {
    println!("Clicked range {range_ix}");
})
```

As part of this, I also added the ability to give text runs a background
color.

Release Notes:

- N/A

crates/editor2/src/editor.rs

@@ -9423,6 +9423,7 @@ impl Render for Editor {
                 font_weight: FontWeight::NORMAL,
                 font_style: FontStyle::Normal,
                 line_height: relative(1.).into(),
+                background_color: None,
                 underline: None,
                 white_space: WhiteSpace::Normal,
             },
@@ -9437,6 +9438,7 @@ impl Render for Editor {
                 font_weight: FontWeight::NORMAL,
                 font_style: FontStyle::Normal,
                 line_height: relative(settings.buffer_line_height.value()),
+                background_color: None,
                 underline: None,
                 white_space: WhiteSpace::Normal,
             },

crates/editor2/src/element.rs

@@ -2452,7 +2452,7 @@ impl LineWithInvisibles {
                         len: line_chunk.len(),
                         font: text_style.font(),
                         color: text_style.color,
-                        background_color: None,
+                        background_color: text_style.background_color,
                         underline: text_style.underline,
                     });