Function resolveEntry

Recursively resolves a config entry or nested config structure:

• Arrays: Each entry is resolved and flat-mapped.
• Objects: If containing "url" and "resolve", the "url" is resolved on the fly; otherwise, recurses into the object.
• Strings:
  • ".txt": Fetches, splits by comma, applies cleanup, and recursively resolves each item.
  • ".json" (excluding "tileset.json"): Fetches and parses as JSON, recurses unless resolve=false.
• Other entries are returned as-is.