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/ui/src/prelude.rs

@@ -11,7 +11,7 @@ pub use crate::clickable::*;
 pub use crate::disableable::*;
 pub use crate::fixed::*;
 pub use crate::selectable::*;
-pub use crate::styles::{vh, vw};
+pub use crate::styles::{rems_from_px, vh, vw};
 pub use crate::visible_on_hover::*;
 pub use crate::{h_flex, v_flex};
 pub use crate::{Button, ButtonSize, ButtonStyle, IconButton, SelectableButton};