Convert Camera to ES6 class syntax and inherit Map from it

[mourner]

I was trying to make Camera a an object used from Map, as discussed in #3408, but it got really messy, with either lots of code duplication or splitting similar responsibilities between Map and Camera, and it also required rewriting tons of camera tests.

So I'm proposing the make Camera the base class of Map instead. This way it looks much cleaner, is a clear improvement over the current state (mixing in Camera.prototype), and doesn't require as many changes. We can follow up on any improvements in subsequent PRs (specifically, balancing out responsibilities between Camera, Transform and Map, and camera event refactoring #3357).

In addition to the change above, I made some minor cleanups in Camera code and also moved all camera events there. will follow up in another PR.

👀 @lucaswoj @jfirebaugh

[mourner]

@jfirebaugh per discussion in chat, rebased the PR to only contain the Camera ES6 class changes so that the diff is easy to read. Will follow-up with other changes in a different PR. Note that I added @memberof Map to all Camera methods so that it still shows up under Map while being inherited (didn't test it though since #3416 needs to be fixed first).

[mourner]

John approved the PR but GH was having issues; merged.