Add GitHub Actions testing, JSHint -> ESLint, rewrite tests in ESM

[mgol]

Also:

• switch to Node 20
• update dependencies

[timmywil]

No need to use a matrix here. I suggest using env to set the node version, setting the workflow name statically. NPM_SCRIPT is already unused below.

[mgol]

I've tried to switch to env and it looks similar to what you did in jQuery UI but it just doesn't work. I have a test PR at mgol#2 and I am getting the following error:

The workflow is not valid. .github/workflows/node.js.yml (Line: 17, Col: 11): Unrecognized named-value: 'env'. Located at position 1 within expression: env.NODE_VERSION

That line is literally:

    name: test (${{ env.NODE_VERSION }})

Any ideas?

[mgol]

Oh; apparently, env variables cannot be used outside steps. I removed the usage from that single line and it's green now: https://github.com/mgol/jquery-ui-themeroller/actions/runs/8584283401/job/23524385015?pr=2