chore: develop on Node 20, CI with 16.17.1 + 18.17.1 + 20 (#23)

This is very similar to [digidem/multi-core-indexer#35][0].

This change updates this project's Node versions:

- Develop on Node 20 by updating `.nvmrc`.

- CI should test on Node 20, 18.17.1, and 16.17.1.

- Require this version in the `engines` field.

This update also causes npm to update, which means that the package
lockfile changed.

Node 16.17.1 and 18.17.1 match versions of nodejs-mobile-react-native,
used by CoMapeo-mobile. nodejs-mobile has the same major & minor
versions as Node, but [different patch versions][1]. For example,
nodejs-mobile-react-native@16.17.10 corresponds to Node 16.17.1.

[0]: digidem/multi-core-indexer#35
[1]: https://github.com/nodejs-mobile/nodejs-mobile-react-native/tree/51afe46d36d3776d1bfe7158073897216b55d7f2#versioning

BREAKING CHANGE: Node 16.17.1+ is now required.

.github/workflows/node.yml

@@ -12,7 +12,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [14.x, 16.x, 18.x]
+        node-version: [16.17.1, 18.17.1, 20.x]
 
     steps:
       - uses: actions/checkout@v2

.npmrc

@@ -0,0 +1 @@
+engine-strict=true

.nvmrc

@@ -1 +1 @@
-14
+20