Remove Pixels: Mul<Pixels, Output = Pixels> impl, add ScaledPixels ops (#27451)

It doesn't make sense to have `Pixels: Mul<Pixels, Output = Pixels>` as
the output should be `Pixels^2` (area), so these impls are removed. All
code where these impls were used are improved by instead multiplying by
`f32` or `usize`.

Also adds math op impls that are present for `Pixels` but absent for
`ScaledPixels`. Adds missing `Mul<Pixels> for usize` to both.

Release Notes:

- N/A

crates/editor/src/editor.rs

@@ -6905,7 +6905,7 @@ impl Editor {
             )
             .child(
                 h_flex()
-                    .h(line_height + BORDER_WIDTH * px(2.))
+                    .h(line_height + BORDER_WIDTH * 2.)
                     .px_1p5()
                     .gap_1()
                     // Workaround: For some reason, there's a gap if we don't do this