fix(mount): pass unknown options to $options on mount

[xanf]

Vue 3.x keeps the behaviour we've had in Vue2: all unknown fields in component are passed as $options. This allows you to do following:

createApp({ ...App, test: 123 }).mount("#app");

and access test on this.$options.test
While not in a great use in modern Vue 3 code, this is still a great importance when we work with @vue/compat - you are still want to be able to pass router/store/etc. in "legacy" way to your component

This PR restores this behavior

[afontcu]

nice catch!