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>

docs/src/configuring-zed.md

@@ -1902,6 +1902,27 @@ Run the `theme selector: toggle` action in the command palette to see a current
 },
 ```
 
+## Unnecessary Code Fade
+
+- Description: How much to fade out unused code.
+- Setting: `unnecessary_code_fade`
+- Default: `0.3`
+
+**Options**
+
+Float values between `0.0` and `0.9`, where:
+
+- `0.0` means no fading (unused code looks the same as used code)
+- `0.9` means maximum fading (unused code is very faint but still visible)
+
+**Example**
+
+```json
+{
+  "unnecessary_code_fade": 0.5
+}
+```
+
 ## An example configuration:
 
 ```json