Fix missing buffer font features in Blame UI, Hover Popover and Markdown Preview (#44657)

- Fix missing font features in 
  `git_ui::blame_ui::GitBlameRenderer.render_blame_entry`
- Fix missing buffer font features in
`markdown_preview::markdown_renderer`
- Update the way that the markdown style is built for hover popovers so
  that, for code blocks, the buffer font features are used.
- Introduce `gpui::Styled.font_features` to allow callers to also set
  the font's features, similar to how `gpui::Styled.font_family` already
  exists.

Relates to #44209

Release Notes:

- Fixed wrong font features in Blame UI, Hover Popover and Markdown
Preview

crates/git_ui/src/blame_ui.rs

@@ -65,7 +65,7 @@ impl BlameRenderer for GitBlameRenderer {
                         .w_full()
                         .gap_2()
                         .justify_between()
-                        .font_family(style.font().family)
+                        .font(style.font())
                         .line_height(style.line_height)
                         .text_color(cx.theme().status().hint)
                         .child(