Split project into modules

[noodlebox]

As this project grows, improved modularity would make it easier to maintain. There are at least four versions of the core emote parsing implementation duplicated across several projects:

• KawaiiDiscord (userscript and BetterDiscord plugin)
• A fork of BetterDiscord with alternative emote parsing
• KawaiiEmotes plugin implementing the same for stock BetterDiscord

Factoring this and a few other things out into separate modules will eliminate the hassle of porting fixes between them, allow building for the different targets from a single set of source files, and allow for automated testing to avoid dumb regressions. Webpack seems like a nice choice of build system, and could also make bundling external libraries cleaner for BD plugins.