Allow custom certificates in HTTPS mode

[albertorestifo]

Thanks to tools like mkcert it's now easier than ever to have a valid HTTPS certificate for local development.

This PR adds new environment variables to allow passing a custom certificate to the Webpack dev server, without introducing any breaking changes:

• HTTPS_CERT: Path to the certificate file
• HTTPS_KEY: Path to the key file

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[akrantz]

#5845 has been sitting around and looks ready to merge in.

[albertorestifo]

Closed in favor of #5845, which is a better and more complete implementation.

Thanks @akrantz for pointing it out.