Provide strong and consistant typing

[wandri]

I've been relying on dagrejs/dagre for a while, which depends on @dagrejs/graphlib. And I have met some typing issues in the last updates.

• For example, children in the the current typing is defined as children(v: string): string[] but in fact it should be children(v?: string): string[];.

I would like a strong typing from Typescript.

I have done a PR for this migration: #223.