dx(runtime-dom): warn config.isCustomElement usage in runtime-only build (#2945)

packages/runtime-core/src/component.ts

@@ -653,6 +653,9 @@ type CompileFunction = (
 
 let compile: CompileFunction | undefined
 
+// dev only
+export const isRuntimeOnly = () => !compile
+
 /**
  * For runtime-dom to register the compiler.
  * Note the exported method uses any to avoid d.ts relying on the compiler types.