Turn this repo into module

[aparlato]

Per previous conversations, we should turn this repo into a module. That way we can use the code here in a standalone diff tool like this repo currently contains and replace this copy-pasta file in Maputnik that allows us to use diffing there. It would also allow us to use the output in a Github action if we wanted this to run on opening PRs.

The copied code in Maputnik uses an additional function to transform the current diff output into something more readable for our purposes. This output format is something we should consider using directly from this module since the current output reflects a diff from a setStyle in GLJS.

[ebrelsford]

If we can, let's try to keep the module setup similar to the build system and the format tool (the latter is probably a better place to start as it's smaller).