feat(compiler): transform slot outlets

packages/runtime-core/src/helpers/renderSlot.ts

@@ -0,0 +1,12 @@
+import { Slot } from '../componentSlots'
+import { VNodeChildren } from '../vnode'
+
+export function renderSlot(
+  slot: Slot | undefined,
+  props: any = {},
+  // this is not a user-facing function, so the fallback is always generated by
+  // the compiler.
+  fallback?: string | VNodeChildren
+): string | VNodeChildren | null {
+  return slot ? slot() : fallback || null
+}