Finalize browser support and polyfill approach

[moodyjmz]

Before releasing 1.0, we need to finalize our browser support matrix and our approach to polyfilling required features (currently ResizeObserver and EventTarget) as needed.

Default assumption is that we'll support current major browsers plus old Edge and IE11, but there is a chance we'd drop support for one or both of those older browsers if performance isn't adequate or if the cost (effort + complexity) of support seems unjustifiably high.