Use javascript instead of coffee-script

[philschatz]

This switches the source code to use .js files instead of .coffee files.

Steps taken so far:

1. decaffeinate src/**.coffee
   • I needed to skip some files because they did not automatically convert
2. replace import X from Y with const X = require(Y)
3. run standard --fix *.js src/**.js to re-lint the files
   • I needed to run eslint directly so I could check that tests pass

TODO

• decide whether to use ES6 features (like const, class) or convert them to vanilla var
• manually convert the remaining src/**.coffee files
  • Note: multiline regular expressions in Coffee-Script are nice because they allow comments
• remove __guard__ function (from decaffeinate)
• convert the test/**.coffee files