fix: no _ssr* in ssr functional tempalte

[clarkdo]

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Other information:

Fix nuxt/nuxt#2565
When use Functional Components in ssr mode, the FunctionalRenderContext which is built in runtime is lack of _ssr* properties(_ssrNode, _ssrList, ...)

[yyx990803]

Thanks for the PR! Since runtime copying for every context creation is non-optimal, I exposed the functional context and now it installs once on both Vue.prototype and the functional context prototype.