Add utility functions for value and class joining

src/core/utils.js

@@ -0,0 +1,5 @@
+export const val = t => typeof t === "function" ? t() : t;
+
+export const joinClass = (t, l) => typeof l === "function" 
+  ? () => `${t} ${l() || ""}`.trim() 
+  : `${t} ${l || ""}`.trim();