Refactor buffer generation script

[alulsh]

This PR refactors the buffer generation script (npm run buffers). It does the following:

• Switches from 4 spaces to 2 spaces for easier reading
• Switches from var to const where possible (ES6 syntax)
• Renames variables to make file name vs. geojson and bike lane vs. county polygon more clear

I am refactoring this to help with a follow up PR to upgrade turf to @turf/turf, which has several breaking changes. Not a straight forward upgrade.

I ran npm run buffers locally and this script worked before and after this refactor.