add: quarto snippets for footnotes and citations

[lasse16]

Footnotes are done in pandoc-style, e.g.

this is a footnote [^identifier]
[^identifier]: with its description here

The identifier must be identical, hence the duplicated tabstop.

[OkelleyDevelopment]

Overall these changes seem good to me. Can you confirm testing of these in your local setup?

If so (and they work 😄), then I will approve and merge!

[OkelleyDevelopment]

@lasse16 following up on my previous question above ^^

[lasse16]

my bad
I lost this PR somewhere in my notifications

The snippets all passed linting and formatting, and did not crash the plugin.
I only tested about 5 of them in my daily work in the last weeks and those were working fine.