Support For define.amd

Trying to use rollup for a refactor of Backburner with @krisselden and others. I noticed that rollup has AMD check looks for `define && define.amd`. Loader.js explicitly opts out of this with `define.petals`, wondering if people would be opposed to just setting `define.amd` to `true`. It's not spec compliant, however loader.js is already not spec compliant in this regard.

Start in Loader.js and inspect how the loader currently exposes define.petals and opts out of the define.amd check. Review the issue discussion before changing compatibility behavior, then verify that Rollup recognizes the loader without breaking its existing AMD behavior. Done means the intended define.amd support is implemented and its compatibility trade-off is covered by the project’s available checks.

Written by the indexing model from the issue text.