Move D&D spell assets into extractor module

internal/modules/extract/dnd/spells/assets/prompts/user.md

@@ -0,0 +1,21 @@
+Source document ID: {{ .SourceID }}
+{{ if .HasChunk }}
+Chunk ID: {{ .ChunkID }}
+Chunk index: {{ .ChunkIndex }}
+{{ end }}
+
+Source units:
+{{ range .Units }}
+- Unit ID: {{ .ID }}
+  Text: {{ .Text }}
+{{ if .Metadata }}
+  Metadata:
+{{ range .Metadata }}
+  - {{ .Key }}: {{ .Value }}
+{{ end }}
+{{ end }}
+{{ end }}
+
+Return only D&D spell-cast artifacts. For each spell cast, identify the in-world
+caster, spell name, effect, narrative description, and source references using
+source_id, start_unit_id, and end_unit_id.