Support for HEREDOC strings

[zharinov]

Perceived Problem

We can't define them at the lexer level because code may contain non-standard begin/end markers.

Ideas / Proposed Solution(s)

These two moo.Lexer methods should be used:

save(): LexerState;
reset(chunk?: string, state?: LexerState): this;

The idea:

• Stop normal processing: call lexer.save() once HEREDOC start is encountered
• Process HEREDOC fragment using custom code or sub-lexer and return single string token as result
• Continue normal processing: call lexer.reset(input, state) with the the input and state values corresponding the position immediately after HEREDOC

[zharinov]

It will be essential for Ruby parser