Support specifying both PHP and JS code in code blocks

[nylen]

The original php-pegjs requires specifying all code inside code blocks as PHP. However, for WordPress/gutenberg we want to use the same PEG in both PHP and JS code (see WordPress/gutenberg#1086).

Accordingly, this PR adds the capability to specify both PHP and JS code in the same code block, using a syntax that is ignored by normal PEG.js, as follows:

{
  /** <?php return $a; ?> **/
  return a;
}

This PR also fixes an instance of void 0 being inserted into the generated PHP code. Elsewhere in generate-bytecode-php.js this is null instead, so the same change has been made here.

[nylen]

@Nordth I am interested in maintaining this library because we'd like to use it in the WordPress project referenced above. Thoughts on this PR and on subsequent changes we will need? (Currently, add an option to avoid PHP namespaces; see WordPress/gutenberg#1152 for details)

[nylen]

I've proceeded with this work at https://github.com/nylen/phpegjs.

[Nordth]

Thanks for sharing! I will recomend to use your fork, because have no time to maintain this project for now