Makefile: Add support to use babel to compile React code using JSX

This is in preparation for starting to use React to implement LMNO
game clients.

In this commit, the previous "make dev" target is now simply "make",
and the previous "make prod" target is now "make LMNO_BUILD=production".
This latter command is definitely annoying to remember, but should
almost never need to be executed manually; instead the developer will
likely invoke "make deploy" which knows how to do a production build.