"missing" EventEmitter props NodeVM inherits

[kylegoetz]

NodeVM extends EventEmitter, so has .on and .off methods.

If you do something like

const vm = new NodeVM({...});
vm.on(ev, handler);

TS will complain that NodeVM has no "on" property because the type definitions for vm2 lacks any indication that NodeVM inherits from EventEmitter. That's what this PR fixes.

[patriksimek]

Thank you!