Add hotkey to navigate active window to nearest data point/polygon

[nwisehart]

Steps to reproduce:

• go to /a11y-map/index.html
• navigate with keyboard to area without pop-ups

Expected result:

• press [INSERT KEY HERE] to move window to nearest data point
• movements should be read into the live region to give user context on where they are being moved
• finished with end result read into live region

Context: In current format, a user with a visual impairment may be unable to navigate to an initial datapoint

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See original at: https://github.com/kellyhutchins/a11y-map/issues/16