feat: add ssrRef capability for automatic SSR support

[danielroe]

When creating composition utility functions, often there will be server-side state that can't currently be conveyed to the client.

This PR adds a drop-in equivalent to ref that will automatically add ref values to window.__NUXT__ - if they have been changed from their initial value. It can be used outside of components, such as in shared utility functions, and it supports passing a factory function that will generate the initial value of the ref.

import { ssrRef } from 'nuxt-composition-api'

const val = ssrRef('')

// When hard-reloaded, `val` will be initialised to 'server-set'
if (process.server) val.value = 'server set'

// When hard-reloaded, the result of myExpensiveSetterFunction() will
// be encoded in nuxtState and used as the initial value of this ref.
// If client-loaded, the setter function will run to come up with initial value.
const val2 = ssrRef(myExpensiveSetterFunction)

closes #20