feat(compiler): ensure interpolation expressions are wrapped with toString()

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

@@ -0,0 +1,10 @@
+import { isArray, isPlainObject, objectToString } from '@vue/shared'
+
+// for conversting {{ interpolation }} values to displayed strings.
+export function toString(val: any): string {
+  return val == null
+    ? ''
+    : isArray(val) || (isPlainObject(val) && val.toString === objectToString)
+      ? JSON.stringify(val, null, 2)
+      : String(val)
+}