Add accessibility feature to animated map

[rachelekm]

Discussed as part of #41, here, the colored states withhold semantic meaning on spending data that has potential to be inaccessible. We should explore and implement an accessibility feature that expands on coloring states.

Some additional context: this includes an exploration phase on what accessibility features currently exist for geojson layers within leaflet components and how it could be implemented as the animated map plays through time. We currently illustrate significant data through updating the feature layers fillColor style, and ways to expand on that could be making the states interactive to use ARIA so data changes can be available to screen readers on state hover or by adding markers or tags. It may not be possible to apply this feature through the play time, in which case we could just apply that feature on the last data step when the time interval completes. Here is a thread on best practices for accessibility in leaflet that may be a helpful starting point.

[mstone121]

Seems like this is blocked on this Leaflet issue being resolved: Accessible vectors #7822. There's a PR open that closes the issue, but it's not clear why it hasn't been merged.