breaking: required props should be inferred, remove @required tag

[metonym]

Fixes #22

#89 added a @required tag to denote a prop as optional. As pointed out in this comment, this is not the correct approach.

Any exported let prop that is uninitialized is required. This can be inferred from the AST.

As a result, @required is not needed. It can even be misleading.

<script>
  // required prop
  export let prop1;
</script>