Added simple 'if' statement support and restructured the implementation.

= mylang features =
* Added simple 'if' statement support.
  - No 'else', etc. clause support.
* Added simple logical expression support.
  - Only can be used as a 'if' statement condition.
  - 'True' and 'False' constants.
  - '&&' (conjunction) and '||' (disjunction) operators.
  - '==' (equals) and '!=' (not equals) boolean comparison operators.

= mylang implementation details =
== Lexer ==
* No more finite state machine for const token (like '&&', '(', etc.) parsing. To modify
const token parsing support, adjust the __const_toks dictionary defined in the constructor.
* Renamed token classes in a more syntax-agnostic fashion.
* Added token classes for the new language features.
* Implemented parsing support for the new language features.
== Parser ==
* Added __try_parse_token and __parse_token parsing support methods.
* Adjusted existing parsing methods with the respect to the added parsing support methods.
* Added node classes for the new language features.
* Implemented parsing support for the new language features.