Fix tests, test more, support Node 16-20

[danyalaytekin]

Changes

• Deps
  • use pa11y-webservice@4.2.0
  • use Mocha 10
  • replace use of obsolete request with node-fetch (can move to native fetch in next major version)
  • add pa11y-lint-config
  • replace underscore with lodash.groupby and lodash.keys
• Building
  • replace Make with npm scripts for everything except less and uglify
  • fix local URLs so they're unaffected by IPV6 issues (arrived with Node 16/18 I think)
• Testing, linting
  • integration tests
    • make them run again, rather than just setup.js
      • also remove a describe.only()
    • fix bugs caused by webservice fixture changes and Cheerio typo
    • assign new DB name to avoid clash for webservice devs (maybe fully randomise this in future PR)
    • detach from config file dependency
    • promisify some parts
  • lint with ESLint and pa11y-lint-config
  • allow ES2019 (can't go higher until drop Node 12)
  • GitHub Actions testing workflow
    • test against Node.js 12, 14, 16, 18, 20 (next major version will drop non-LTS)
    • add MongoDB compatibility tests (independently of those now present in pa11y-webservice, so that this project can test its own stated support)
    • extract linting into its own job
    • bump actions deps
    • replace npm install with npm ci
    • wait for pa11y-webservice to be available, rather than sleeping for 10s
• Dev help
  • supply .nvmrc (went with 14 because of Apple Silicon)
• Update docs
  • report these changes
  • add new support policy
  • tidy up

[danyalaytekin]

Some build errors after updating pa11y-webservice to 4.3.0... checking.

[danyalaytekin]

I've fixed poor spelling of lodash.groupby both in this repo and in pa11y-webservice:

• https://github.com/pa11y/pa11y-webservice/pull/157

Fixing pa11y-webservice back to ~4.2.0 for now even though npm weirdness might have allowed it through.

[danyalaytekin]

pa11y-webservice@4.3.1 is now in npm. This one hasn't been reviewed yet so I'll try it here, and will close draft 319 if all goes well:

• 319

[danyalaytekin]

^ It works!

[danyalaytekin]

Thanks @hollsk for the review. Re: linter warnings, these ones seem pretty valid. I don't think the lines under test were changed by this PR, but I'll create a separate PR to separate them into different lines and quieten those nags.