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/assistant/src/prompt_library.rs

@@ -926,6 +926,7 @@ impl PromptLibrary {
                                                         color: Some(cx.theme().status().predictive),
                                                         ..HighlightStyle::default()
                                                     },
+                                                    ..EditorStyle::default()
                                                 },
                                             )),
                                     ),