Move endsWith() to UtilityClass.cpp and put it in header file

This ensures that endsWith() can be used outside of editor.cpp.

When leo60228 originally wrote endsWith(), it was static, but I asked
him on Discord just now and he more-or-less confirmed that it's fine if
it's not static. If it was static, it would be confined to
UtilityClass.cpp now instead!

desktop_version/src/UtilityClass.cpp

@@ -230,3 +230,16 @@ bool is_positive_num(const std::string& str, bool hex)
 	}
 	return true;
 }
+
+bool endsWith(const std::string& str, const std::string& suffix)
+{
+	if (str.size() < suffix.size())
+	{
+		return false;
+	}
+	return str.compare(
+		str.size() - suffix.size(),
+		suffix.size(),
+		suffix
+	) == 0;
+}