[CS2] Fix #4209: --require for filenames that are invalid identifiers

[GeoffreyBooth]

Fixes #4209. I hate that this adds 15K to the size of the compiler, but fortunately the increase is only in the Node version, not the browser version (which doesn’t include command.coffee).

Previously --require hello-world.coffee would fail because the compiler was prepending hello-world = require('hello-world.coffee');. Now the compiler checks that hello-world is a valid identifier before outputting it; if it’s invalid, the compiler just outputs require('hello-world.coffee') without the assignment.

[lydell]

There are other places where we (in theory) need to check for valid identifiers as well, but, after much discussion, chose not to include that crazy regex.

Can't we output something like global['hello-world'] = require('hello-world.coffee'); instead?

[GeoffreyBooth]

D’uh. That’s a much better solution.

[lydell]

Well, I can't think of anything breaking because of this.

[jashkenas]

Whew!