Encode rem values derived from pixels using rems_from_px

[maxdeviant]

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 rust-lang/rust#57241.

Release Notes:

• N/A