feat(core): add missing styles warning

packages/core/src/utils/errors.ts

@@ -1,4 +1,5 @@
 export enum ErrorCode {
+  MISSING_STYLES = 'MISSING_STYLES',
   MISSING_VIEWPORT_DIMENSIONS = 'MISSING_VIEWPORT_DIMENSIONS',
   NODE_INVALID = 'NODE_INVALID',
   NODE_NOT_FOUND = 'NODE_NOT_FOUND',
@@ -16,6 +17,8 @@ export enum ErrorCode {
 }
 
 const messages = {
+  [ErrorCode.MISSING_STYLES]: () =>
+    `It seems that you haven't loaded the necessary styles. Please import '@vue-flow/core/dist/style.css' to ensure that the graph is rendered correctly`,
   [ErrorCode.MISSING_VIEWPORT_DIMENSIONS]: () => 'The Vue Flow parent container needs a width and a height to render the graph',
   [ErrorCode.NODE_INVALID]: (id?: string) => `Node is invalid\nNode: ${id}`,
   [ErrorCode.NODE_NOT_FOUND]: (id: string | null) => `Node not found\nNode: ${id}`,

packages/core/src/utils/log.ts

@@ -1,7 +1,11 @@
 const productionEnvs = ['production', 'prod']
 
 export function warn(message: string, ...args: any[]) {
-  if (!productionEnvs.includes(__ENV__ || '')) {
+  if (isDev()) {
     console.warn(`[Vue Flow]: ${message}`, ...args)
   }
 }
+
+export function isDev() {
+  return !productionEnvs.includes(__ENV__ || '')
+}