bug: transformation removes async keyword from function declaration

[boromisp]

async is removed with the export:

- export async function f() {
+ function f() {
  await fetch("https://example.com");
}

export async function g() {
  await f();
}

---

It also removes unused positional function arguments in some cases it shouldn't.
Here is a contrived example of that:

function f(
  options: Partial<{
    fn: (url: string, unused: number, qs: string) => string;
  }>
) {
  return options.fn?.("https://example.com", 123, "q=1");
}

export function g() {
  return f({
-    fn(url, unused, qs) {
+    fn(url, qs) {
      return url + (qs.length > 0 ? `?${qs}` : "");
    },
  });
}

[kazushisan]

@boromisp thank you! I will work on a fix!

[kazushisan]

I will separate the latter issue to #28