src/node/util.ts: Make certificate generation "modern"

Now we add a subject alt name, set extendedKeyUsage and use the
correct certificate extension.

The above allow it to be properly trusted by iOS.

See https://support.apple.com/en-us/HT210176

*.cert isn't a real extension for certificates, *.crt is correct
for it to be recognized by e.g. keychain or when importing as a profile
into iOS.

Updates #1566

I've been able to successfully connect from my iPad Pro now to my
code-server instance with a self signed certificate! Next commit
will be docs.