Make unnecessary code fade configurable (#14442)

This PR allows configuring the intensity of code fade applied to unused
code (relates to #4785).

_Note: Right now I included it as a top level config which might be a
little out of place (but is easiest to instrument). Open for suggestions
on where else it would be better suited for._

_Note 2: I am unfamiliar with the codebase. Feel free to either close
this PR and re-implement in a better way or suggest high level changes
if I'm approaching anything wrong :)._

Release Notes:

- Added `unnecesary_code_fade` setting to control how strongly to fade
unused code.

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/editor/src/display_map.rs

@@ -74,8 +74,6 @@ pub enum FoldStatus {
 
 pub type RenderFoldToggle = Arc<dyn Fn(FoldStatus, &mut WindowContext) -> AnyElement>;
 
-const UNNECESSARY_CODE_FADE: f32 = 0.3;
-
 pub trait ToDisplayPoint {
     fn to_display_point(&self, map: &DisplaySnapshot) -> DisplayPoint;
 }
@@ -691,7 +689,7 @@ impl DisplaySnapshot {
             let mut diagnostic_highlight = HighlightStyle::default();
 
             if chunk.is_unnecessary {
-                diagnostic_highlight.fade_out = Some(UNNECESSARY_CODE_FADE);
+                diagnostic_highlight.fade_out = Some(editor_style.unnecessary_code_fade);
             }
 
             if let Some(severity) = chunk.diagnostic_severity {