Add empty npmignore so that lib files get published

[NullVoxPopuli]

tested with npm pack

[ef4]

Thanks, this is almost right but closing in favor of #194 because that one avoids publishing the .ts file (which can troll people if you include it, since it can take precedence over the .js file and cause people to try to compile your TS under a typescript version you didn't expect, causing type errors).