refactor(code): Move zotero live citations to quarto ext

Instead of it lying in the src directory (where, reasonably, only python
data extraction, processing and modelling code should lie), I turned
the retorque filter into a simple quarto extension.
Could reasonably be repackaged separately from this repo since I believe
other people could profit off it.

_extensions/pandoc-to-zotero-live/_extension.yml

@@ -0,0 +1,7 @@
+title: pandoc-to-zotero-live
+author: Emiliano Heyns
+maintainer: Marty Oehme
+version: 0.1.0
+contributes:
+  filters:
+    - pandoc-to-zotero-live.lua

_quarto.yml

@@ -23,7 +23,7 @@ format:
     number-sections: true
     reference-doc: 02-data/supplementary/justified.docx
     filters:
-       - src/pandoc-to-zotero-live.lua
+       - pandoc-to-zotero-live
   pdf:
     echo: false
     number-sections: true