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/ui/src/components/keybinding.rs

@@ -160,7 +160,7 @@ impl RenderOnce for Key {
                 }
             })
             .h(rems_from_px(14.))
-            .text_ui()
+            .text_ui(cx)
             .line_height(relative(1.))
             .text_color(cx.theme().colors().text_muted)
             .child(self.key.clone())