Move DnD shared prompt assets into helper package

internal/modules/sharedassets/dnd/assets/prompts/common-dnd-references.md

@@ -0,0 +1,12 @@
+Optional reference material for this Dungeons & Dragons campaign is provided
+below. Use it only to disambiguate names, aliases, speakers, campaign terms, or
+spell names already present in the transcript.
+
+Player list reference:
+{{ input "players" }}
+
+Party roster reference:
+{{ input "party" }}
+
+Glossary reference:
+{{ input "glossary" }}