Support Embroider

[simonihmig]

There were two issues here how the FeatureParser generated imports, that somehow worked in a classic build, but broke in Embroider:

• the default export of yadda-annotations.js was imported, though that doesn't even exist, it only has named exports. Probably the way ember-cli transpiles ES modules to AMD made this work accidentally!?
• the import path of the steps file was something like tests/accpetance/steps/foo-steps.js, which is neither an absolute path (there is no tests package) nor a relative. Changed to use relative paths (./steps/foo-steps.js)

Also adds additional test scenarios for Embroider, using their new test infrastructure.

This is based on top of #83, as it would have merge conflicts otherwise, so best review by commit.

[simonihmig]

Tests failing due to embroider-build/embroider#587, but was passing locally when manually installing the embroider dependencies rather through the ember-try helpers.