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/theme/src/settings.rs

@@ -91,6 +91,7 @@ pub struct ThemeSettings {
     pub active_theme: Arc<Theme>,
     pub theme_overrides: Option<ThemeStyleContent>,
     pub ui_density: UiDensity,
+    pub unnecessary_code_fade: f32,
 }
 
 impl ThemeSettings {
@@ -283,6 +284,10 @@ pub struct ThemeSettingsContent {
     #[serde(rename = "unstable.ui_density", default)]
     pub ui_density: Option<UiDensity>,
 
+    /// How much to fade out unused code.
+    #[serde(default)]
+    pub unnecessary_code_fade: Option<f32>,
+
     /// EXPERIMENTAL: Overrides for the current theme.
     ///
     /// These values will override the ones on the current theme specified in `theme`.
@@ -550,6 +555,7 @@ impl settings::Settings for ThemeSettings {
                 .unwrap(),
             theme_overrides: None,
             ui_density: defaults.ui_density.unwrap_or(UiDensity::Default),
+            unnecessary_code_fade: defaults.unnecessary_code_fade.unwrap_or(0.0),
         };
 
         for value in sources.user.into_iter().chain(sources.release_channel) {
@@ -602,6 +608,10 @@ impl settings::Settings for ThemeSettings {
                 value.buffer_font_size.map(Into::into),
             );
             merge(&mut this.buffer_line_height, value.buffer_line_height);
+
+            // Clamp the `unnecessary_code_fade` to ensure text can't disappear entirely.
+            merge(&mut this.unnecessary_code_fade, value.unnecessary_code_fade);
+            this.unnecessary_code_fade = this.unnecessary_code_fade.clamp(0.0, 0.9);
         }
 
         Ok(this)