Switch from karma to testem; better broccoli integration

[mnutt]

This is a rather hefty PR that:

• upgrades ember to 3.6
• upgrades a lot of other dependencies
• replaces rollup.config.js with build.js, a broccoli-based builder
• compiles tests/tests.js so that imports work properly
• does a better job of parsing index.js such that multi-line imports now work
• gets sourcemaps working!
• continues to support existing apps still using rollup.config.js
• gives more useful error messages if an app does not compile properly