[v3]: Indicate supported Node.js versions; test on Node.js v16

[legobeat]

• Targeting Version 3 #125

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• Indicate supported Node.js versions via engines.node in manifest
• ci: Test on node.js v16

[paulmillr]

Can we use same engines.node that noble curves / hashes use?

[legobeat]

Can we use same engines.node that noble curves / hashes use?

For sure. I was thinking it's a nice property that each supported major version is also tested in CI, which reduces the likelihood of gaps. And thinking there is little value in maintained support for odd-versioned (non-LTS) majors 19/21.

Requiring higher minor versions of the legacy versions also means we can fearlessly use newer language- and runtime features on 16.x series that may not be available in 16.0.0. Since CI is typically running on latest 16.x. For this reason, I now also added CI running explicitly on the lowest supported versions as well (16.0.0 and 18.0.0).

In any case, updated according to suggestion.