refactor: use const instead of enum of one member

Co-authored-by: Sebastian Good <2230835+scagood@users.noreply.github.com>

Author: RebeccaStevens

lib/rules/no-sync.js

@@ -45,10 +45,7 @@ module.exports = {
                                 {
                                     type: "object",
                                     properties: {
-                                        from: {
-                                            type: "string",
-                                            enum: ["file"],
-                                        },
+                                        from: { const: "file" },
                                         path: {
                                             type: "string",
                                         },