Added Non-Associative associativity for Pratt Parsers.

[YunkaiZhang233]

In current Pratt parsers, only associativity type of left and right are supported. However, according to the original designs of Pratt parsing, it should also support Non-Associative infix binary operators.

For example, consider operators == as a non-associative operator. We should be able to identify it as non-associative and therefore rejecting 1 == 2 == 3 as an expression with the numbers being atomic literals. Whilst the current design only allow == to be either left-associative or right-associative.

This PR added associativity non for infix binary operators, and changed the binding power calculation function slightly based on The Original Parsing Methodology and this article.