Add getClientRects and getBoundingClientRect stubs to Range

[jeffrifwald]

Adds Rect methods to Range following the current implementation of the Element.prototype.getBoundingClientRect and Element.prototype.getClientRects stubs.

Should address #3002

[domenic]

As discussed in #2751, we're not clear on the right path for these sorts of stubs, and it'd be better to address them holistically.

Additionally, we'd want to introduce proper implementations of DOMRect, instead of using POJSOs.

So, if you're interested in pursuing this, I'd suggest:

• Doing a separate PR which implements DOMRect support. (I might try to do this today by extracting the relevant pieces of the mega-PR SVG improvements #2926.)
• Helping drive the discussion in Fix for TypeError: range(...).getBoundingClientRect is not a function #3002 by identifying an exhaustive list of all stub methods that are desired, and whether or not jsdom is currently stubbing them.

I'm sorry that this is more work than you might have anticipated.