In "Fill match arms", allow users to prefer Self to the enum name when possible

But default to not to.

I chose to have a more generic config name because maybe other assists could also use the same approach.

editors/code/package.json

@@ -680,6 +680,16 @@
                     }
                 }
             },
+            {
+                "title": "assist",
+                "properties": {
+                    "rust-analyzer.assist.preferSelf": {
+                        "markdownDescription": "When inserting a type (e.g. in \"fill match arms\" assist), prefer to use `Self` over the type name where possible.",
+                        "default": false,
+                        "type": "boolean"
+                    }
+                }
+            },
             {
                 "title": "assist",
                 "properties": {