Implement the "this" keyword

[Razican]

The this keyword returns an expression dependent on the context where it's being used. This context is resolved by the ResolveThisBinding() abstract operation. This abstract operation will get the current environment record, and return the this binding, if any.

The MDN documentation also explains how this keyword will resolve to a different expression depending on how it's called.

To implement this, we require changes to the parser, where keywords are parsed (parser.rs#L112), which will probably require a new ExprDef. Executing that expression in the executor, it should return the object representing the this binding.

[jasonwilliams]

Thanks for making this

[Razican]

I just noticed that #185 exists.

[jasonwilliams]

it does but its quite an old pr now