Improve local link checking.

[Mehgugs]

Adds some npm boilerplate to make local link checking easier.

[advaith1]

changing "npm install" to run a different command seems like a bad idea

[splatterxl]

it doesn't run a different command it, just runs alongside the normal package install, although I'm not sure about which goes first; "postinstall" would be better here for both semantics and ensuring it runs after install

[shaydewael]

I like the idea of making it easier to test links locally, but I agree with @advaith1 that npm install shouldn't be modified here. Also with this, npm run build would be run twice in the Github workflow.

For this kinda thing, could see a pre-commit or pre-push hook being helpful, but that may just be overdoing it. we can also just add instruction on the readme to run npm run build && npm run test:links without modifying the package.json scripts.

[appellation]

Closing since this appears to be stale