feat(compiler-core): export `transformElement` from compiler-core (#907)

packages/compiler-core/src/index.ts

@@ -48,6 +48,6 @@ export {
   trackVForSlotScopes,
   trackSlotScopes
 } from './transforms/vSlot'
-export { resolveComponentType, buildProps } from './transforms/transformElement'
+export { transformElement, resolveComponentType, buildProps } from './transforms/transformElement'
 export { processSlotOutlet } from './transforms/transformSlotOutlet'
 export { generateCodeFrame } from '@vue/shared'