Add ui_font_size setting (#3199)

This PR adds a new `ui_font_size` setting that can be used to control
the scale of the entire UI.

We use the value in this setting to set the base rem size of the window.

Release Notes:

- N/A

crates/ui2/src/prelude.rs

@@ -4,21 +4,12 @@ pub use gpui2::{
 };
 
 pub use crate::elevation::*;
-use crate::settings::user_settings;
 pub use crate::ButtonVariant;
 pub use theme2::ActiveTheme;
 
-use gpui2::{rems, Hsla, Rems};
+use gpui2::Hsla;
 use strum::EnumIter;
 
-pub fn ui_size(cx: &mut WindowContext, size: f32) -> Rems {
-    const UI_SCALE_RATIO: f32 = 0.875;
-
-    let settings = user_settings(cx);
-
-    rems(*settings.ui_scale * UI_SCALE_RATIO * size)
-}
-
 #[derive(Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy, EnumIter)]
 pub enum FileSystemStatus {
     #[default]