Always use Unix-style line endings in generator output

[nbes4]

This PR adds:

• A centralized serialize() implementation. I think this one made sense since it was the same for Device, Symbol, Package and Component. If these implementations differ in the future we could remove it again, but for now it prevents some code duplication. (I wasn't sure how to name it and where to put it, I opted for serialize_common and put it in the outer common.py file, suggestions welcome)
• Always use Unix-style line endings (LF) when serializing entities (should fix development on Windows, see discussion here)
• Tests

I tried the librepcb-cli open-library --all --strict --check --minify-step . command before the changes in this PR and it gave me the "Non-canonical file". After applying the changes in this PR it seemed to work just fine! :-)

Edit: I verified the Tests on my Windows machine. They failed before applying the changes, but passed after applying the changes from this PR.

[nbes4]

@dbrgn Thanks for the review, I implemented your feedback!

Did you test whether the test actually fails when running on Windows without the fix?

Yes, I forgot to mention that but I did run the tests on my Windows machine before applying the changes in this PR and they failed, but passed once I applied the changes 👍 I updated the initial PR description to make it more clear upfront 😄

[nbes4]

Thanks for merging 🎉

@rnestler more coverage never hurts IMO 👍