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/popover_menu.rs

@@ -1,13 +1,13 @@
 use std::{cell::RefCell, rc::Rc};
 
 use gpui::{
-    div, overlay, point, prelude::FluentBuilder, px, rems, AnchorCorner, AnyElement, Bounds,
+    div, overlay, point, prelude::FluentBuilder, px, AnchorCorner, AnyElement, Bounds,
     DismissEvent, DispatchPhase, Element, ElementContext, ElementId, HitboxId, InteractiveElement,
     IntoElement, LayoutId, ManagedView, MouseDownEvent, ParentElement, Pixels, Point, View,
     VisualContext, WindowContext,
 };
 
-use crate::{Clickable, Selectable};
+use crate::prelude::*;
 
 pub trait PopoverTrigger: IntoElement + Clickable + Selectable + 'static {}
 
@@ -102,7 +102,7 @@ impl<M: ManagedView> PopoverMenu<M> {
     fn resolved_offset(&self, cx: &WindowContext) -> Point<Pixels> {
         self.offset.unwrap_or_else(|| {
             // Default offset = 4px padding + 1px border
-            let offset = rems(5. / 16.) * cx.rem_size();
+            let offset = rems_from_px(5.) * cx.rem_size();
             match self.anchor {
                 AnchorCorner::TopRight | AnchorCorner::BottomRight => point(offset, px(0.)),
                 AnchorCorner::TopLeft | AnchorCorner::BottomLeft => point(-offset, px(0.)),