Constraint checks

[jchavarri]

This PR adds:

• some initial checking of installed npm packages versus the constraints found on opam depexts file that include the npm-version variable
• some basic error handling
• tests for the above

I think this is enough to release a version to opam, the only remaining thing would be to lift the OCaml compiler constraint so that the opam plugin also works with 5.0.

[jchavarri]

One remaining thing I'm not that happy about is that the plugin will check for node_modules from the folder where it's called from:

https://github.com/jchavarri/opam-check-npm-deps/blob/cfe4d54f5215fcd8c13807503c2ad028a9cc7bd6/src/main.ml#L113

Maybe this is fine for now, and we can think about smarter ways to find the node_modules folder in the future.

[davesnx]

Sounds good for a 0.0.1. Possible ways to improve:

• Adding a --root flag
• A mechanism for looking into git folder
• Recursively going up for a node_modules directory

[jchavarri]

@davesnx @anmonteiro thanks for the reviews. I think I tackled all the comments, I will merge, then work on testing OCaml 5, and then cut a first release to submit to opam.

A couple of issues that originated from this PR have been created:

• Support for monorepos / multiple versions of packages #5
• Should the plugin leverage opam formulas for npm-version constraints? #4