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/extensions_ui/src/extensions_ui.rs

@@ -521,7 +521,7 @@ impl ExtensionsPage {
                     .gap_2()
                     .border_1()
                     .border_color(editor_border)
-                    .min_w(rems(384. / 16.))
+                    .min_w(rems_from_px(384.))
                     .rounded_lg()
                     .child(Icon::new(IconName::MagnifyingGlass))
                     .child(self.render_text_input(&self.query_editor, cx)),