Rework build scripts to avoid the use of gulp

[arbron]

Remove gulp and related dependencies to avoid outstanding security vulnerabilities and clean up the CLI a bit.

[Varriount]

Just curious, why?

[arbron]

Just curious, why?

Vulnerabilities with Gulp dependencies. Since it hasn't had any new releases since 2019 the chance of those being fixed doesn't seem high.

[Fyorl]

I just want to note that, since gulp is a dev dependency, users are not exposed to any of these vulnerabilities, so it is not a super high priority or anything. It does mean there's a fair bit of noise when running npm audit though, meaning we could miss more serious vulnerabilities. Since we can replace gulp fairly easily in this case then we might as well.

[Fyorl]

One additional comment is that there seems to have been a critical vulnerability raised against @babele/traverse so feel free to bump some dependency versions additionally, or fix it with npm audit before we merge this.