refactor: Replace cached hyphenator function with static member variable

lib/Epub/Epub/hyphenation/Hyphenator.h

@@ -4,6 +4,8 @@
 #include <string>
 #include <vector>
 
+class LanguageHyphenator;
+
 class Hyphenator {
  public:
   struct BreakInfo {
@@ -16,4 +18,7 @@ class Hyphenator {
 
   // Provide a publication-level language hint (e.g. "en", "en-US", "ru") used to select hyphenation rules.
   static void setPreferredLanguage(const std::string& lang);
+
+ private:
+  static const LanguageHyphenator* cachedHyphenator_;
 };