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/chat_panel/message_editor.rs

@@ -18,7 +18,7 @@ use project::{search::SearchQuery, Completion};
 use settings::Settings;
 use std::{ops::Range, sync::Arc, time::Duration};
 use theme::ThemeSettings;
-use ui::{prelude::*, UiTextSize};
+use ui::{prelude::*, TextSize};
 
 use crate::panel_settings::MessageEditorSettings;
 
@@ -523,7 +523,7 @@ impl Render for MessageEditor {
             },
             font_family: settings.ui_font.family.clone(),
             font_features: settings.ui_font.features,
-            font_size: UiTextSize::Small.rems().into(),
+            font_size: TextSize::Small.rems(cx).into(),
             font_weight: FontWeight::NORMAL,
             font_style: FontStyle::Normal,
             line_height: relative(1.3),