feat: add citation element

[mhatzl]

This PR adds the citation element, and CSL handling to render citations according to a given CSL style.

Relevant decisions you made in this PR

Storing citation IDs in parser contexts

According to CSL, citations of the same IDs may differ depending on the citation order.
To correctly handle this, all citations are first stored in a citation list
that is stored in the BlockContext of the block parser.
Before rendering to an output format, the parsed citations are processed in-order by citeproc-js
to get the correct citation content.
The rendered citations are then passed as list to the respective renderer for the output format.

CSL locale files in preamble

CSL processors need locale information to correctly render citations.
The official CSL repository provides some locales, but not every locale is covered.
Therefore, the parameter citation_locales was added to allow adding custom locale files.

[mhatzl]

It is still a draft, so take my comments more like hints and suggestions.

Summary:

• Added some remarks to reduce possible panics.
• Suggested using more functionality provided by serde
• Suggested workaround to get a HashMap for clap + serde

[mhatzl]

looks good.
One minor change to use Option<String> to prevent "" from being used as None.

I tried to check the JS part, but I am not too familiar with JS.
Could not find any obvious errors, and the tests are passing, so I assume that its ok.