Can I still code in C, but use Espruino as a JS interpriter?

At the moment about the best you could do is write a library as @oesterle suggests and then add your code to run in the main loop on an idle handler - and you probably want to compile with GCC and a Makefile too since it needs a few Python scripts to precalculate tables.

Espruino ends up being quite tightly integrated to the MCU - handling sleep, interrupts, and having its own IO queues - it's basically a mini OS. While with some work could compile it to run under something else, suddenly you'd miss out on all the power saving, bluetooth, GPIO, etc.

There have been questions about that in the past, and it could be done - however there's very little incentive for me to put effort into making it easy to do since it's not likely to sell any more Espruino boards. I imagine by far the biggest interest would be from people using it on their own hardware.