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
404adbce5b