✨ Improve type definition of `parse` function

[micalevisk]

Now users can optionally supply the type of the parsed data like the following:

const myData = toml.parse<MyData>(myInput);
//        ^? MyData

instead of doing

const myData: MyData = toml.parse(myInput);

Notice that the default type is still any thus both approach above works.

---

As an workaround to this, I augmented toml with this ambient file:

toml.d.ts

```ts declare module 'toml' { export function parse(input: string): TOutput; } ```

[micalevisk]

Hi @BinaryMuse! can you please review this one? it should be pretty straightforward :smiley:

[micalevisk]

I fond that this is a bad style. We should return unknown instead