Remove python from the js compilation

[restingbull]

The python executable in the js can be replaced by a starlark rule with minimal fuss, and removes python dependency from the rule set.

[TLATER]

I've been looking at this; it's a bit of a fuss because it of course still needs to run through an external binary, but unzip and jar support regex extraction as well, so it seems feasible. Currently figuring out how the tests work in this repository, but I'll get a PR up asap :)

@ulfjack and @Yannic may be interested too