[feature]: add server prefetch hook

[skyline0705]

Add serverPrefetch hook for vue-function-api.
In my project, we use this hook for SSR. I have tried it and looks work fine~~

[pi0]

Any updates on this?

/cc @LinusBorg @posva @liximomo @TooBug

[posva]

I imagine the PR would need a test.
Since SSR is yet to be added to https://vue-composition-api-rfc.netlify.com/api.html, I'm unsure about. To me it makes sense for this library to implement it as it's for 2.x anyway and also list it on the Readme

[rebeccarich]

Any update on this one @posva @LinusBorg @liximomo @TooBug? :)
Would be great to get an idea of the roadmap for SSR in this plugin is before we commit to using it in our app. SSR is a requirement for us unfortunately

[michael-zucchetta]

This would be really helpful for us too and I think it should be added

[pi0]

I think we should also ensure no context sharing is happening during concurrent SSR requests. runtimeContext depends on a global reference!

[filrak]

Any updates on that? I would happily commit my time to push it forward if needed. This feature is a must-have for our project unfortunately ;/

[Akryum]

@pi0 I'm not sure this would be a problem, the global variables are used only during setup as it's processing the different components. So for a different request it will be overridden by each component one at a time.

[Akryum]

Thanks for the PR @skyline0705 ! Closing in favor of #198