Update index.d.ts

[albancreton]

RE: #80

Updating DOMRectReadOnly based on the declaration in typescript's lib.dom.

[garronej]

go go go

[vilbergs]

Seems correct according to https://github.com/microsoft/TypeScript/blob/master/lib/lib.dom.d.ts

[vilbergs]

It seems that ResizeObserverEntry is out of sync as well, it's missing the following properties. Perhaps this PR could incorporate that.

interface ResizeObserverEntry {
    readonly borderBoxSize: ReadonlyArray<ResizeObserverSize>;
    readonly contentBoxSize: ReadonlyArray<ResizeObserverSize>;
    ...
}

EDIT:

My bad... These are not implemented in the package yet, as per #77

[cascornelissen]

Pinging @que-etc on the off change that he still logs into his account regularly/reads his email. We would love to get this merged and published as it's blocking in one of our projects.

[davidnormo]

I've emailed @que-etc directly incase he's disabled his github email notifications