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/editor/src/hover_popover.rs

@@ -623,7 +623,10 @@ pub fn hover_markdown_style(window: &Window, cx: &App) -> MarkdownStyle {
     });
     MarkdownStyle {
         base_text_style,
-        code_block: StyleRefinement::default().my(rems(1.)).font_buffer(cx),
+        code_block: StyleRefinement::default()
+            .my(rems(1.))
+            .font_buffer(cx)
+            .font_features(buffer_font_features.clone()),
         inline_code: TextStyleRefinement {
             background_color: Some(cx.theme().colors().background),
             font_family: Some(buffer_font_family),