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/prelude.rs

@@ -11,7 +11,7 @@ pub use crate::clickable::*;
 pub use crate::disableable::*;
 pub use crate::fixed::*;
 pub use crate::selectable::*;
-pub use crate::styles::{rems_from_px, vh, vw, PlatformStyle};
+pub use crate::styles::{rems_from_px, vh, vw, PlatformStyle, StyledTypography};
 pub use crate::visible_on_hover::*;
 pub use crate::{h_flex, v_flex};
 pub use crate::{Button, ButtonSize, ButtonStyle, IconButton, SelectableButton};