Add Vue watch mixin to update a variable on changes to another.

To be used where adding .sync to props is not convenient for
one reason or another.

lib/www/client/source/src/lib/watcher-mixin.js

@@ -0,0 +1,14 @@
+import { deepCompare } from './utils';
+
+function setIfDifferent(propsLocals) {
+  return Object.fromEntries(Object.entries(propsLocals).map( ([prop, local]) => [
+    local,
+    () => {
+      if (!deepCompare(this[prop], this[local])){
+        this[local] = structuredClone(this[prop]);
+      }
+    }
+  ]));
+}
+
+export default setIfDifferent;