Implement property accessors

[tofpie]

ECMASCript feature
I would like to see the property accessors parsing and execution implemented. ECMAScript specification.

Example code
This code should now work and give the expected result:

let a = { get b() { return "c"}}
a.b

The expected output is "c".

[RageKnify]

This is tied to #577, @HalidOdat was leading development on that, sadly you implemented it on Value in #987 , which would require us to change it all once we want to move the behaviour to GcObject...
@HalidOdat what are your thoughts?