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/styles/typography.rs

@@ -4,6 +4,8 @@ use gpui::{
 use settings::Settings;
 use theme::{ActiveTheme, ThemeSettings};
 
+use crate::rems_from_px;
+
 #[derive(Debug, Default, Clone)]
 pub enum UiTextSize {
     /// The default size for UI text.
@@ -38,10 +40,10 @@ pub enum UiTextSize {
 impl UiTextSize {
     pub fn rems(self) -> Rems {
         match self {
-            Self::Large => rems(16. / 16.),
-            Self::Default => rems(14. / 16.),
-            Self::Small => rems(12. / 16.),
-            Self::XSmall => rems(10. / 16.),
+            Self::Large => rems_from_px(16.),
+            Self::Default => rems_from_px(14.),
+            Self::Small => rems_from_px(12.),
+            Self::XSmall => rems_from_px(10.),
         }
     }
 }