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/outline/src/outline.rs

@@ -306,7 +306,7 @@ impl PickerDelegate for OutlineViewDelegate {
                 .selected(selected)
                 .child(
                     div()
-                        .text_ui()
+                        .text_ui(cx)
                         .pl(rems(outline_item.depth as f32))
                         .child(styled_text),
                 ),