dx(compiler-core): warn on <template v-for> key misplacement

Note: the behavior is different from Vue 2. `<template v-for>` are compiled
into an array of Fragment vnodes so the key should be placed the `<template>`
for v-for to use it for diffing.

packages/compiler-core/__tests__/transforms/vFor.spec.ts

@@ -263,6 +263,34 @@ describe('compiler: v-for', () => {
         })
       )
     })
+
+    test('<template v-for> key placement', () => {
+      const onError = jest.fn()
+      parseWithForTransform(
+        `
+      <template v-for="item in items">
+        <div :key="item.id"/>
+      </template>`,
+        { onError }
+      )
+
+      expect(onError).toHaveBeenCalledTimes(1)
+      expect(onError).toHaveBeenCalledWith(
+        expect.objectContaining({
+          code: ErrorCodes.X_V_FOR_TEMPLATE_KEY_PLACEMENT
+        })
+      )
+
+      // should not warn on nested v-for keys
+      parseWithForTransform(
+        `
+      <template v-for="item in items">
+        <div v-for="c in item.children" :key="c.id"/>
+      </template>`,
+        { onError }
+      )
+      expect(onError).toHaveBeenCalledTimes(1)
+    })
   })
 
   describe('source location', () => {