Added support for @code ... @endocde commands

[nikarh]

Adds support for doxygen code commands

Notable changes:

• Added \t char as a possible whitespace
• Whitespaces used to be collapsed in the lexer. Since code blocks should retain all formatting, lexer emits Whitespace items for every whitespace instead. Now they are collapsed in the parser, which checks if the last GrammarItem ends with a whitespace
• @code can optionally have an additional parameter (lang) passed with brackets as in @code{.py}. For that, I changed the parameter parsing logic to parse parameters separated with whitespace (including multiple whitespace lexemes) and inside of the brackets.