feat: add RTL support in epub and txt readers (#1700)

Co-authored-by: Zach Nelson <zach@zdnelson.com>

lib/Utf8/Utf8.h

@@ -5,6 +5,8 @@
 #define REPLACEMENT_GLYPH 0xFFFD
 
 uint32_t utf8NextCodepoint(const unsigned char** string);
+// Appends a Unicode codepoint to a std::string in UTF-8 encoding.
+void utf8AppendCodepoint(uint32_t cp, std::string& out);
 // Remove the last UTF-8 codepoint from a std::string and return the new size.
 size_t utf8RemoveLastChar(std::string& str);
 // Truncate string by removing N UTF-8 codepoints from the end.