feat: add @vue/compat initial support

[xanf]

Vue 3.1 brings compat build, which is extremely important milestone in migrating huge codebases to Vue3.
@vue/test-utils plays surprisingly well with compat build and codebase in compat mode with only few changes required in code

Why changes are required? Migration guide suggests enabling features via configureCompat as part of certain migration steps. If RENDER_FUNCTION feature is enabled, signature of render function will be similar to Vue2, so our stubs will not render slots. Luckily, we can override it, by explicitly setting compatConfig on our stub components (this has no effect if no compat build is in play).

Additionally a few magic hops are required for example for finding components declared with Vue.extend.

Please, take a note - this is not a complete changes list (for example setValue should take into account that model event might be set back to legacy "input" if COMPONENT_V_MODEL flag is in play), but is a foundation introducing testing compat build to CI and a couple of fixes

[lmiller1990]

Awesome, great to hear things mostly work with the compat build. Just one question!

[lmiller1990]

I'm not super familiar with how the compat build works - what is SubVue? Where does this come from?

[xanf]

It comes from here: https://github.com/vuejs/vue-next/blob/51d2be20386d4dc59006d31a1cc96676871027ce/packages/runtime-core/src/compat/global.ts#L229

Basically as a result of Vue.extend in compat build you will always have function with name SubVue

Unfortunately there is no 100% reliable way to check that "component is coming from legacy extend" but I find SubVue name matching reliable enough to work in 99.9% cases (come-on, if you name your functional component SubVue you're in trouble :))

Forget about it. Found a better way to build assertion, thank you for your question

[lmiller1990]

Nice