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/collab_ui/src/collab_panel/channel_modal.rs

@@ -174,7 +174,7 @@ impl Render for ChannelModal {
                     .child(
                         h_flex()
                             .w_full()
-                            .h(rems(22. / 16.))
+                            .h(rems_from_px(22.))
                             .justify_between()
                             .line_height(rems(1.25))
                             .child(CheckboxWithLabel::new(