Add docstring and sigil as per Erlang/OTP 27

Erlang/OTP 27 introduces two new language elements [1], triple-quoted
strings (docstrings) [2] and sigils (type prefix for string literals)
[3].

[1] https://www.erlang.org/news/168
[2] https://www.erlang.org/docs/27/system/data_types.html#string
[3] https://www.erlang.org/docs/27/system/data_types.html#sigil

Author: Kornel Horvath