feat(cli): make scan usable with npx command

[tamascsaba]

Description

Add bin to package.json to make to be compatible with the npx command, enabling users to run the scan tool directly from the command line.

Motivation

I would like to run observatory locally or in CI :)

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[argl]

Thanks for taking the time to submit your PR, very much appreciated. I tested it out and unfortunately it did not work on my system. But the issue is very much valid, so I did this PR today, which basically does the same thing. Let me know if this fixes your issues.

[tamascsaba]

The fix is perfect, I haven't had time to test it. 👍

npm supports github repository so it is possible to run with this command:

npx mdn/mdn-http-observatory mdn.dev

but It would be really great, if you could publish this package to the npm registry as well.