Add Test Runner for Node 18 LTS

[OneCricketeer]

README says LTS Node versions are supported. Yet, Node 18 LTS is untested for pull-requests.

Describe the solution you would like to see
Would like to see Node v18 tests running and passing.

Additional context
Maybe 12 should be removed? https://nodejs.dev/en/about/releases/

[lholmquist]

Maybe 12 should be removed?

Totally agree. Node 14 also goes EOL at the end of April, so maybe we should wait until then, so we only have to do 1 major version bump.