Update (dev) dependencies flagged by npm-audit

[rgrunber]

• mocha from 8.1.1 to 9.1.3
• ts-loader from 5.3.1 to 9.2.6
• Update compatible dependencies of gulp 4.0.2

[rgrunber]

The remaining dependencies flagged by npm audit come from gulp, and we are at the latest version, 4.0.2. Though we update the underlying dependencies, if compatible, I think we have cases where the version of a dep in which the problem is fixed might be a major bump, whereas the dependency is only listed as compatible with gulp for minor bumps, so not sure if we can fix it without a new gulp version.

Articles like https://overreacted.io/npm-audit-broken-by-design/ are an interesting read.