feat: add renderToString function

It is now possible to use `renderToString` to test the SSR rendering of a component.

To do so, use `renderToString(MyComponent)`.
`renderToString` returns a `Promise<string>` with the HTML rendered.

```ts
  it('returns correct html with pre-fetched data on server', async () => {
    const Component = defineComponent({
      template: '<div>{{ text }}</div>',
      setup() {
        const text = ref<string | null>(null)
        onServerPrefetch(async () => {
          text.value = await fakeFetch('onServerPrefetch')
        })
        return { text }
      }
    })

    const contents = await renderToString(Component)

    expect(contents).toBe('<div>onServerPrefetch</div>')
  })
```

docs/guide/advanced/ssr.md

@@ -0,0 +1,40 @@
+# Testing Server-side Rendering
+
+Vue Test Utils provides `renderToString` to test Vue applications that use server-side rendering (SSR).
+This guide will walk you through the process of testing a Vue application that uses SSR.
+
+## `renderToString`
+
+`renderToString` is a function that renders a Vue component to a string.
+It is an asynchronous function that returns a Promise,
+and accepts the same parameters as `mount` or `shallowMount`.
+
+Let's consider a simple component that uses the `onServerPrefetch` hook:
+
+```ts
+function fakeFetch(text: string) {
+  return Promise.resolve(text)
+}
+
+const Component = defineComponent({
+  template: '<div>{{ text }}</div>',
+  setup() {
+    const text = ref<string | null>(null)
+
+    onServerPrefetch(async () => {
+      text.value = await fakeFetch('onServerPrefetch')
+    })
+
+    return { text }
+  }
+})
+```
+
+You can write a test for this component using `renderToString`:
+
+```ts
+it('renders the value returned by onServerPrefetch', async () => {
+  const contents = await renderToString(Component)
+  expect(contents).toBe('<div>onServerPrefetch</div>')
+})
+```