Which programming environments or APIs are available for the LEGO BOOST?
Boost and the other Powered Up components (the new City trains, the app-controlled batmobile, and WeDo 2.0) are controlled via Bluetooth. Any environment that can send bluetooth commands can be used to control these components.

There is a github page which documents some of the reverse-engineering that has been done and it links to several third-party Boost/PoweredUp programming libraries. I've used movehub, but there is another node.js library called node-poweredup which looks more complete.

Lego themselves have documented part of the BLE wireless protocol on github as well.