Update support policy, tooling, and docs

[danyalaytekin]

This PR goes some way to update the tooling used in this project:

1. Replace Grunt tasks with npm scripts
2. Replace JSHint with ESLint and pa11y-lint-config
3. FIx lockfile version to 2
4. Publish with minimum supported version of Node (18)

It also:

1. Documents how to use nektos/act to validate/test the workflows. This is a simpler project than most of the others so act works fine here.
2. Adds our chosen support policy.

[hollsk]

Alright this looks good to me. I note your question about LGPL vs GPL, and to the best of my knowledge, LGPL is the right one. I'm just going to approve this PR, and we can just merge and then rationalise the licensing later (might need to go through more repos to see if there are more inconsistencies, so makes sense to me to do it in a batch)

Thanks!!