feature: default value for string parameter

```ts
export async function main(
  a: number = 3,
  b: string = "hi"
) {
  // ...
}
```

In this case, `a` would be `3` when a user does not input from UI.

However, `b` will be `""` (empty string), not `"hi"`, when a user does not input from UI.

IMHO, respecting a default value will be suitable for 99% cases.

What do you think?
