Method append_char

* append_char and flush_str are used to modify entities in PHP source code like string literals * e.g. we have to tokenize this code: $x = "New\n"; * but in class-field `std::string SrcFile::text` we have R"($x = \"New\\n\";)" * we will replace "\\n" with "\n" and get string_view on this representation from text field * * It's really strange behaviour to modify source text, it's better to hav