Handle keys dropped from JSON-LD document during expansion (closes #50)

[anatoly-scherbakov]

• Add submodules
• Update .gitignore
• Use spec tests from git modules as default test targets
• list[…] → List[…] because this form is for Python ⩾ 3.10 only
• Note about Git submodules in README.rst
• Document addition of on_key_dropped argument and its behavior
• Implement on_key_dropped argument
• Adapt tests by @pchampin to cover dropped keys handler

[BigBlueHat]

@anatoly-scherbakov can you rebase this on the main branch, so it doesn't include the #182 code?

[anatoly-scherbakov]

@BigBlueHat that's the reason why it's in Draft :(

It should be rebased upon #182 but Github won't allow me to do that; I think #182 has to be merged first. I will do my subsequent PRs in a proper fork of the repo. Currently, as it seems, I am using some sort of hidden fork that Github automatically creates when you try to direct your PR to upstream repo you do not control, I am not sure what exactly is going on with it.

[mielvds]

@anatoly-scherbakov fyi, I included a stripped down version of your implementation here to #228