refactor: adjust internal vnode types + more dts tests

test-dts/index.d.ts

@@ -1 +1,7 @@
+// This directory contains a number of d.ts assertions using tsd:
+// https://github.com/SamVerschueren/tsd
+// The tests checks type errors and will probably show up red in VSCode, and
+// it's intended. We cannot use directives like @ts-ignore or @ts-nocheck since
+// that would suppress the errors that should be caught.
+
 export * from '@vue/runtime-dom'