[Intelligent code completion] AssignmentsFinder shouldn't index variables when it can't infer the proper type

[karreiro]

AssignmentsFinder shouldn't suggest attributes for target in the following scenario, because we can't guess the correct type:

{%- liquid
  if use_variant
    assign target = cart
  else
    assign target = product
  endif
%}
{{ target.█ }}

Relates: #663