Include AST in result of svelte.compile

[Rich-Harris]

For cases like sveltejs/v2.svelte.dev#103, it would be useful to have access to the component AST. Should probably sanitize it first (so that internal change-prone things like _scope don't accidentally become depended on) which would be relatively straightforward — if it turns out that has a non-trivial impact on speed (which I doubt) then it could be put behind an option.

const { code, map, ast } = svelte.compile(source);