fix support indexed accessor properties in Object Literals

[p-bakker]

Adds support for indexed accessor properties.

Without this, running the the current master of Test262 hangs on https://github.com/tc39/test262/blob/main/test/built-ins/Array/prototype/concat/arg-length-near-integer-limit.js, as the getter for index 0 is never called

Closes #906
Closes #775

[gbrail]

Thanks for the timely fix!

This method has a ton of undocumented prerequisites that predate your fix -- it assumes that the index is either a String or an Integer, for example. I think that's fine since it's tightly embedded with the interpreter and code generation.