Break typography styles out of StyledExt (#11013)

- Centralizes typography-related UI styles and methods in
`styles/typography.rs`
- Breaks the typography-related styles out of `StyledExt`. This means we
add a `StyledTypography` trait – this should more or less be an
invisible change as we publish it in the prelude.
- adds the ability to easily grab the UI or Buffer font sizes
(`ui_font_size`, `buffer_font_size`) with `TextSize::UI`,
`TextSize::Editor`

Release Notes:

- N/A

crates/collab_ui/src/notifications/collab_notification.rs

@@ -34,7 +34,7 @@ impl ParentElement for CollabNotification {
 impl RenderOnce for CollabNotification {
     fn render(self, cx: &mut WindowContext) -> impl IntoElement {
         h_flex()
-            .text_ui()
+            .text_ui(cx)
             .justify_between()
             .size_full()
             .overflow_hidden()