Add support for the `{% liquid %}` tag

[charlespwd]

In this PR

• Splits Helpers out of the LiquidHTML ohm grammar
• Splits Liquid <: Helpers out of the LiquidHTML ohm grammar
• Adds a new ohm grammar, LiquidStatement <: Liquid, with slightly different rules:
  • space is /[ \t]/ instead of /[ \t\r\n]/
  • liquidTagRule is $name $tagMarkup(?=newline) instead of {%-? $name $tagMarkup -?%}
• Means we now have 4 grammars:
  • Helpers
  • Liquid <: Helpers
  • LiquidStatement <: Liquid
  • LiquidHTML <: Liquid
• Means we have a generic Liquid grammar we could use for non-HTML cases (!).
• Adds support for the {% liquid %} tag

Decisions:

• At most one empty line between statements
• No leading empty lines
• No trailing empty lines
• Indent as you would
• Print other tags as you would, but replace new lines with spaces
• %} on a new line, like the other tags

Examples

It should parse tags properly and print them properly
printWidth: 1
{% liquid
  assign x = thing.property | filter1 | fitler2
  echo x | filter1 | filter2
  capture var
    if null
      echo true
    else
      echo false
    endif
  endcapture
  cycle 'one', 'two', 'three'
  for x in col reversed limit: 10
    echo x
  else
    echo 'nope'
  endfor

  form 'cart', cart
    echo 'huh?'
  endform

  increment i
  section 'string'
  layout 'string'
  paginate col by 10, window_size: 50
    render 'icon' with x as bar, foo: bar
  endpaginate
%}

Fixes #6