Add a test suite and converting the existing test files to the new test suite

[romainmenke]

fixes: #8

This is based on #25
But all changes to grammars/css.cson from that branch have been reverted to ensure that we add tests first, before making changes.

[romainmenke]

see: https://github.com/microsoft/vscode/blob/main/.editorconfig

[romainmenke]

Roughly based on https://github.com/microsoft/vscode/blob/main/.github/workflows/ci.yml

[romainmenke]

See: https://github.com/microsoft/vscode/blob/main/.nvmrc

[romainmenke]

I've converted the previous test suite from coffeescript to plain JavaScript.
This should make it more approachable for others to add and update tests.

[romainmenke]

Mutations:

• CoffeeScript -> JavaScript
• node:test as the test runner
• node:assert as the assertion lib

There are 7 failing test cases.
I've skipped these for now with .skip

[romainmenke]

@aeschli, @andreamah could you take a look at this proposed change when you have time please? 🙇 Thank you in advance.

[romainmenke]

@aeschli Sorry to keep pinging you on this and please do forward if someone else should follow up. I just happen to know that you also work on https://github.com/microsoft/vscode-css-languageservice.

What would be the best way to make progress on this issue and to start making regular improvements to this package?