[@vue/compat] Prevent local asset registration from affecting other local apps (fix #5699)

[snoozbuster]

Closes #5699. Because applySingletonAppMutations copies the reference of the singleton context instead of a copy of it, all local asset registration into any local app modifies all other local apps both post-facto and globally. My initial take at solving this was to simply create a shallow copy of the singleton app context instead of copying the reference, but then I realized the existing implementation allows for code like the following to work:

const app = createApp({ template: '<foo />' });
Vue.component('foo', /* some impl */);
app.mount(...); // resolves 'foo' from the global registry even though it was created after the fact

I'm not sure if this is behavior we wanted or meant to have, but in the interest of not breaking anything, I added a test for both the original issue and this one, and fixed both by using a proxy setup which first attempts to resolve components from the singletonApp registry, and if that doesn't work it instead resolves them from the local one. This implementation passes all tests. (If we're not interested in preserving the old behavior of allowing post-facto global registrations to affect local apps, then we can strip off the latter two commits and use the much simpler shallow copy implementation).

[yyx990803]

I simplified to logic to still pass the test.

Note that:

• In the mixins case, the proxy won't be able to mimic the same behavior as objects. Currently it's a simple clone so it won't cover late-applied global mixins for now.

• This actually doesn't fix [vue/compat] createApp copy singletonApp configs even set { mode:3 } to compatConfig  #5699 - it's a separate bug. See 2f07e34

[snoozbuster]

oh nice. using the singleton app's context as the prototype makes for a pretty natural solution. looking forward to the release for this!