bundle index.d.ts

[jnaglick]

right now we distribute individual .d.ts files (1:1 with src) files. This (1) works fine (2) is very straight-forward because the files are an direct artifact of the bundling process (3) is probably meaningless or might even be preferably from a dev-user's perspective

HOWEVER it seems a lot of projects just distribute one "bundled" index.d.ts. So, maybe we can add https://github.com/dmitrytut/rollup-plugin-dts-bundle to do this.

NOTE: Do NOT use https://www.npmjs.com/package/rollup-plugin-dts, which is different, and appears to have a bug with composite build strategies that wont be fixed.