Rework build system

[jkillian]

The build flow is now TS [-> via TypeScript ->] ES6 [-> via Babel ->] ES5

There are a few benefits to this commit:

• It fixes Fix Interop with CommonJS Usage #760. Why? ES6 modules export their default content as a member called default. When we switched over in Modularize #726 to ES6 modules we introduced this problem for CommonJS consumers. Babel has a special feature that fixes this. So, we use TS then Babel to get the result we want.
• The compilation now uses our tsconfig.json files to determine compilation options. This makes things more consistent across IDE and builds, there's no longer two separate sets of files and options to maintain.
• Related to the previous point, our build now updates the files field of our tsconfig.json files automatically. No more manually adding files or adding files through a special IDE feature! This means that only filesGlob should be adjusted manually.

There are a few other minor tweaks, but the points above are the major parts of this PR

[jkillian]

Let's use #764 and #765 instead of introducing Babel.