npm v8 support

[yorickvP]

Fixes #33

• Added CI with tests for npm v6, npm v8 and yarn
• Also rewrite new package-lock.json format
• Add a check for unsupported package-lock.json format
• Add a passthru.nodeModules attribute, very useful for debugging
• Persist the cache in the nodeModules drv. This is kindof ugly, but npm prune sometimes installs packages now :(.

@gromakovsky please forward this for review to whoever is using this internally

[balsoft]

Maybe also remove the travis badge from README

[yorickvP]

Just pushed some changes that greatly simplify the cache building (it no longer has to rewrite the lockfile).

Also submitted npm/cacache#114 to be more efficient about putting the npm cache in the nix store.

[balsoft]

Overall pretty clean, and seems to work! I like it

[gromakovsky]

We'll test it in our project, thanks!