Make exponentiation operator right-associative

[aspeddro]

JS exponentiation operator is right-associative: a ** b ** c is equal to a ** (b ** c), but in ReScript is left-associative.

let a = 2. ** 3. ** 2.

var a = Math.pow(Math.pow(2, 3), 2);

This PR is a proposal to make it right-associative.

var a = Math.pow(2, Math.pow(3, 2));

Forum Post

[cristianoc]

Would you change the printer too, and add tests to res_syntax/tests/printer/expr

[aspeddro]

Would you change the printer too, and add tests to res_syntax/tests/printer/expr

Done

[cristianoc]

This looks great.
Thanks a lot for the contribution.