Include expectedProperties in stats

[Rich-Harris]

My strategy for v3: Add a v3 command to svelte-upgrade, then use it to update all the tests so we don't have to do it manually. Side-benefit: it ensures we actually get around to making svelte-upgrade work for the 2-3 migration (to the extent possible, at least — I expect some unavoidable breakage).

Once the tests are done, it'll be much easier to gradually implement the RFC.

A key thing we need for svelte-upgrade is to be able to know which properties are expected by the component (either because they're referenced by computed properties, or they're referenced in the markup and are not computed properties).

The plan:

const { ast, stats } = svelte.compile(`
<h1>Hello {name}!</h1>

<script>
  export default {...};
</script>
`, {
  generate: false
});

assert.deepEqual(stats.props, ['name']);

It's a shame we can't distinguish between props and state — we'll have to treat everything as a prop (i.e. add export for everything) for correctness. But that's just one of many ways in which v3 will be better than v2.