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/elements/label.rs

@@ -1,4 +1,4 @@
-use gpui2::{relative, Hsla, WindowContext};
+use gpui2::{relative, rems, Hsla, WindowContext};
 use smallvec::SmallVec;
 
 use crate::prelude::*;
@@ -86,7 +86,7 @@ impl Label {
                         .bg(LabelColor::Hidden.hsla(cx)),
                 )
             })
-            .text_size(ui_size(cx, 1.))
+            .text_size(rems(1.))
             .when(self.line_height_style == LineHeightStyle::UILabel, |this| {
                 this.line_height(relative(1.))
             })