Allow variable identifiers to be conditionals

This aligns with the Python behaviour and pre-existing Jinja behaviour
where a variable expression can be used as a test for a conditional.
This was necessary to allow variable identifiers to be used in places
which was expecting a possible conditional expression, like in a if/else
expression.

grammar.ebnf

@@ -222,6 +222,7 @@ conditional_expression
     | conditional_expression_logical
     | conditional_expression_comparator
     | conditional_expression_test
+    | variable_identifier
     ;
 
 conditional_expression_if