feat: v-once

Note: only compiler transform is tested - integration with runtime
still needs to be tested.

packages/compiler-core/src/transforms/vOnce.ts

@@ -0,0 +1,15 @@
+import {
+  DirectiveTransform,
+  createObjectProperty,
+  createSimpleExpression
+} from '@vue/compiler-core'
+
+export const transformOnce: DirectiveTransform = dir => {
+  return {
+    props: createObjectProperty(
+      createSimpleExpression(`$once`, true, dir.loc),
+      createSimpleExpression('true', false)
+    ),
+    needRuntime: false
+  }
+}