Add CI testing for Node 18, drop Node 10 and 12

[ericyhwang]

Node 12 reached end-of-life on 2022-04-30, and Node 18 was promoted to an active release as of 2022-04-19.

sharedb@3 does the same thing, adding Node 18 and dropping Node 12.

[coveralls]

Coverage decreased (-100.0%) to 0.0% when pulling b8a8cee on node-18 into 60e0c84 on master.

[coveralls]

Coverage decreased (-100.0%) to 0.0% when pulling b8a8cee on node-18 into 60e0c84 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling b8a8cee on node-18 into 60e0c84 on master.