fix: nullable return types of HTMLElement

[wdzeng]

Hi,

This is the greatest tool I've ever found. But some types flaw with TypeScript.

Following methods in HTMLElement can return null, but do not declare correct return types.

• getElementById
• querySelector (in README only)
• nextSibling
• nextElementSibling
• previousSibling
• previousElementSibling

Following methods in HTMLElement can return undefined, but do not declare correct return types. Actually these should follow the web API and return null, but I did not change them to prevent breaking the backward compability.

• getAttribute (in README and jsdocs only)
• firstChild
• lastChild