Encode rem values derived from pixels using rems_from_px (#9367)

This PR adds a new `rems_from_px` helper function that can be used to
compute rem values based on a pixel value.

This is something we do fairly commonly, where we want to express a size
that is a given pixel size at the base rem size (e.g., "14px when the
rem size is 16px").

`rems_from_px` helps make the intent more explicit, as well as prevent
the base rem size from being duplicated everywhere.

Note: Ideally we would want `rems_from_px` to be `const`, but that
depends on https://github.com/rust-lang/rust/issues/57241.

Release Notes:

- N/A

crates/breadcrumbs/src/breadcrumbs.rs

@@ -86,7 +86,7 @@ impl Render for Breadcrumbs {
             ),
             None => element
                 // Match the height of the `ButtonLike` in the other arm.
-                .h(rems(22. / 16.))
+                .h(rems_from_px(22.))
                 .child(breadcrumbs_stack),
         }
     }