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]

@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.