Support type variable for documents in TypeScript

[hackerwins]

Description:

Change the SDK interface so that TypeScript users can use the type system better.

type Model = {
  k1: Array<number>;
  k2: yorkie.Array<string>;
  k3: {key: string};
  k4: yorkie.Object<{key: string}>;
  k5: yorkie.Text;
  k6: yorkie.RichText;
}

const doc = new yorkie.Document<Model>('doc-key');
doc.update((root) => {
  root.k1 = [1,2,3];
  root.k2 = new yorkie.Array<string>();
  root.k3 = {key: 'abcd'};
  root.k4 = new yorkie.Object<{key: string}>({key: 'abcd'});
  root.k5 = new yorkie.Text();
  root.k6 = new yorkie.RichText();
});

This issue is a follow-up to #289..

Why:

• TypeScript users can use the type system better

[hackerwins]

This was resolved by #299, #300.