Tweak isUnusedOrUnnecessary

The first cut was a bit rough with the blanket `unused_*` rule. This
trigger for things like `unused_mut` where the code is used but it's
suboptimal. It's misleading to grey out the code in those cases.
Instead, use an explicit list of things known to be dead code.

editors/code/src/utils/rust_diagnostics.ts

@@ -95,8 +95,14 @@ function isUnusedOrUnnecessary(rd: RustDiagnostic): boolean {
         return false;
     }
 
-    const { code } = rd.code;
-    return code.startsWith('unused_') || code === 'dead_code';
+    return [
+        'dead_code',
+        'unknown_lints',
+        'unused_attributes',
+        'unused_imports',
+        'unused_macros',
+        'unused_variables'
+    ].includes(rd.code.code);
 }
 
 /**