Introduce "depcheck" to clean up npm dependencies

[macbre]

Depcheck is a tool for analyzing the dependencies in a project to see: how each dependency is used, which dependencies are useless, and which dependencies are missing from package.json.

Before

$ npx depcheck
Unused dependencies
* ansicolors
* ansistyles
* ascii-table
* async
* csv-string
* node-statsd
Unused devDependencies
* mockery
Missing dependencies
* @jest/globals: ./test/bin-program.test.js