Liquid: update for 5.0.0

[EricFromCanada]

Now that Liquid 5.0.0 is out, this is a rework of the Liquid lexer for simplification and additional syntax features (including a few that aren't in the documentation yet):

• liquid tag for enclosing multiple tags within one set of delimiters
• as in Render and Include tags
• forloop.parentloop reference
• ranges can be assigned to variables
• continue keyword for forloop offset parameter
• mark delimiters as Comment::Preproc (as is done in php.rb, jinja.rb, etc.)
• adding a mimetype
• addressing some corner cases, which have been added to the sample file
• add support for the proposed inline comment syntax