Fix lookup on variable with literal name

[dylanahsmith]

Problem

#60 introduced a subtle regression for doing a lookup on a variable with a name that matches a literal.

For example,

Liquid::Template.parse("{{ blank.svg | asset_url }}", error_mode: :strict)

would result in the following error after that pull request was introduced

Liquid::SyntaxError (Liquid syntax error: Expected end_of_string but found dot in "{{ blank.svg | asset_url }}")

but we would get the strict parse error on the commit before that PR was merged or when just using the liquid gem.

This quirk comes from the LITERALS.key?(markup) check being used to detect if the markup is for a literal. If that literal name is followed by an object lookup (e.g. .svg) then it is handled by VariableLookup which will treat the literal name (e.g. blank) as the variable name.

Solution

Lookahead to make sure the identifier isn't followed by a . or [ token before treating it as a literal.