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/components/icon.rs

@@ -1,4 +1,4 @@
-use gpui::{rems, svg, IntoElement, Rems};
+use gpui::{svg, IntoElement, Rems};
 use strum::EnumIter;
 
 use crate::prelude::*;
@@ -15,10 +15,10 @@ pub enum IconSize {
 impl IconSize {
     pub fn rems(self) -> Rems {
         match self {
-            IconSize::Indicator => rems(10. / 16.),
-            IconSize::XSmall => rems(12. / 16.),
-            IconSize::Small => rems(14. / 16.),
-            IconSize::Medium => rems(16. / 16.),
+            IconSize::Indicator => rems_from_px(10.),
+            IconSize::XSmall => rems_from_px(12.),
+            IconSize::Small => rems_from_px(14.),
+            IconSize::Medium => rems_from_px(16.),
         }
     }
 }