feat: Extract author from XTC/XTCH files (#563)

## Summary

* Extract author from XTC/XTCH files

## Additional Context

* Based on updated details in
https://gist.github.com/CrazyCoder/b125f26d6987c0620058249f59f1327d

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### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? No

lib/Xtc/Xtc.h

@@ -56,6 +56,7 @@ class Xtc {
 
   // Metadata
   std::string getTitle() const;
+  std::string getAuthor() const;
   bool hasChapters() const;
   const std::vector<xtc::ChapterInfo>& getChapters() const;