Improve LCP accuracy by ignoring some clicks/keypresses

[smhmic]

Right now the library allows inert clicks and keypresses (i.e. user clicks on 'loading' gif) to skew LCP scores lower. Two ideas for mitigating this:

1. Do not allow user clicks and keypresses to deactivate LCP monitoring until FID.
2. Offer a way to customize which inputs/elements deactivate LCP monitoring.

[philipwalton]

This library adds a click and keydown listener just as a means of reporting the "final" LCP value sooner. It does this because the underlying Largest Contentful Paint API itself will stop listening for new largest paints after user input or scrolls (see #75 to know why this library stopped listening for the scroll event).

If w3c/largest-contentful-paint#43 in the LCP spec is addressed, this library will not have to use the click and keydown listeners, and it can just rely on the underlying API to know when observation has stopped.

[philipwalton]

I'm going to close this issue since I don't think there's any action this library needs to take at this time.