add new tag - parseAssign

[sanket0896]

This MR adds a new tag to our fork of LiquidJS. THE TAG IS called parseAssign.

It converts array or objects passed as a string to their JS object value and assigns these values to the key provided. This key is then appended to the Scope.

{% parseAssign arr = "[1,2,3]" %}
{% parseAssign obj = '{"prop1": prop1Val, "prop2": prop2Val}' %}

Result:
Scope now has

{
    ...,
    arr: [1,2,3],
    obj: {
        prop1: prop1Val,
        prop2: prop2Val
    }
}

Note: The value being assigned should be valid JSON. Invalid JSON will results in exception being thrown.

Passes tests

[sanket0896]

Note: Please squash and merge