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.

packages/runtime-core/src/index.ts

@@ -87,7 +87,7 @@ export {
   resolveDynamicComponent
 } from './helpers/resolveAssets'
 // For integration with runtime compiler
-export { registerRuntimeCompiler } from './component'
+export { registerRuntimeCompiler, isRuntimeOnly } from './component'
 export {
   useTransitionState,
   resolveTransitionHooks,