chore: update contributing instructions

[pfitzseb]

Fixes #273.

cc @mitiemann

[mitiemann]

I've ran npm run test on my WSL Ubuntu 22.04 (the current default afaik) which got my a SyntaxError.
npm ci produced

npm WARN EBADENGINE   package: 'mocha@10.2.0',
npm WARN EBADENGINE   required: { node: '>= 14.0.0' },
npm WARN EBADENGINE   current: { node: 'v12.22.9', npm: '8.5.1' }
npm WARN EBADENGINE }

added 116 packages, and audited 117 packages in 1s

25 packages are looking for funding
  run `npm fund` for details

1 high severity vulnerability

To address all issues, run:
  npm audit fix

Run `npm audit` for details.

mtiemann@devicename:~/dev/atom-language-julia$ npm --version
8.5.1
mtiemann@devicename:~/dev/atom-language-julia$ node --version
v12.22.9

I've installed nodejs and npm via apt. I'll find a suitable way to install a more recent version and get back to you.

[mitiemann]

Looking good