Make code generation consistent with `tsc`

[redallen]

Currently, react-tokens, react-styles, and react-icons all generate their JS/TS differently from one another. Additionally, their output does not match what tsc would generate from similar source files.

react-icons has the additional issue that there are a few static files related to icon creation that must be compiled in addition to dynamically generating the code for each icon.

Bonus points for unifying the templating system that's currently a combination of interpolation, plop, and a custom API like this:

module.exports = {
  getOutputPath:
  getContent:
  getSingleOutputPath:
  getSingleContent:
};

Towards #3972