Codechange: let ReadLine return a string instead of passing a buffer

src/strgen/strgen.h

@@ -67,11 +67,9 @@ struct StringReader {
 
 	/**
 	 * Read a single line from the source of strings.
-	 * @param buffer The buffer to read the data in to.
-	 * @param last   The last element in the buffer.
-	 * @return The buffer, or nullptr if at the end of the file.
+	 * @return The line, or std::nullopt if at the end of the file.
 	 */
-	virtual char *ReadLine(char *buffer, const char *last) = 0;
+	virtual std::optional<std::string> ReadLine() = 0;
 
 	/**
 	 * Handle the pragma of the file.