document Marker#getElement and add Popup#getElement

[mollymerp]

per a question on stackoverflow, Marker#getElement was not documented, and I also noticed Popup did not have an equivalent function.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page

[anandthakker]

🤔 there's a a slight mismatch here between the element returned by getElement() -- which is the wrapping <div class="mapboxgl-popup-content">...</div> -- versus the element provided by, say, setDOMContent() / setHTML() / setText(), which would set the content of that div.

If someone does setHTML(<div class="my-popup">...</div>), they might reasonably expect to get the "my-popup" div back from getElement(). But since the the content provided by the setters need not be a single element, we can't use that as the return value for getElement(). Maybe the mismatch is fine, but we specify it in the docs?

[mollymerp]

ah you're right @anandthakker – it is weird especially bc this will include the little close button we include by default 💭

[mollymerp]

removing Popup portion of this PR – will revisit if we get user requests.