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/editor.rs

@@ -384,6 +384,7 @@ pub struct EditorStyle {
     pub status: StatusColors,
     pub inlay_hints_style: HighlightStyle,
     pub suggestions_style: HighlightStyle,
+    pub unnecessary_code_fade: f32,
 }
 
 impl Default for EditorStyle {
@@ -400,6 +401,7 @@ impl Default for EditorStyle {
             status: StatusColors::dark(),
             inlay_hints_style: HighlightStyle::default(),
             suggestions_style: HighlightStyle::default(),
+            unnecessary_code_fade: Default::default(),
         }
     }
 }
@@ -9698,6 +9700,7 @@ impl Editor {
                                                     color: Some(cx.theme().status().predictive),
                                                     ..HighlightStyle::default()
                                                 },
+                                                ..EditorStyle::default()
                                             },
                                         ))
                                         .into_any_element()
@@ -12588,6 +12591,7 @@ impl Render for Editor {
                     color: Some(cx.theme().status().predictive),
                     ..HighlightStyle::default()
                 },
+                unnecessary_code_fade: ThemeSettings::get_global(cx).unnecessary_code_fade,
             },
         )
     }