Add Lua flashcards deck

Wire the Lua deck into the per-language and combined build pipelines and
the CI matrix so cards.md is built into cards.apkg, cards-with-name.apkg,
and cards.pdf alongside the other languages.

Author: ad-si

.github/workflows/build-artifacts.yml

@@ -20,6 +20,7 @@ jobs:
           - godot
           - sqlite
           - wolfram-language
+          - lua
     steps:
       - name: Checkout
         uses: actions/checkout@v6

lua/.gitignore

@@ -0,0 +1,3 @@
+cards-test
+cards.apkg
+cards.pdf