Extend Citavi6 translator to import BibTex-Keys

[JAC28]

• Workaround to work with syntax of Better BibTex plugin
• Writes Keys into Extra-Field
• Provided Sample-Data contains Demo-Project with auto-generated Keys. First entry is without Key for testing.

[JAC28]

Reason for the extension:

When transferring from Citavi to Zotero, the existing BibTex keys are not transferred, if I see it correctly. Using the Better BibTex plug-in ensures that the keys are re-issued, which means that compatibility with previous work based on the Citavi BibTex keys is lost.
If the extra fields contain a Citation Key: BibTeyKey pair, Better BibTex will adopt it. This ensures backwards compatibility of the keys even after transfer to Zotero.

[adam3smith]

Something weird happening with the commit -- looks like you modified every line (EOL character or indenting, I assume). I'd guess linting locally would prevent that.

[JAC28]

I'll create a new PR. I think the scaffold tool has added the testcases to the translator file.