Add getOwnPropertyDescriptor Proxy trap

[svaarala]

Add minimal support for getOwnPropertyDescriptor Proxy trap to support virtualized enumeration.

Tasks:

• Add Proxy passthrough for reading a descriptor when a getOwnPropertyDescriptor trap doesn't exist
• Add Proxy passthrough for Object.defineProperty() and variants
• Add getOwnPropertyDescriptor trap call, minimally to handling enumeration at least
• Check propdesc call sites; especiall e_idx + a_idx assumptions
• Test coverage for passthrough
• Test coverage for trap call
• Specific test for virtualized enumeration
• Releases entry

[svaarala]

One internal difficulty with this change is that most internals deal with duk_propdesc struct which provides property attributes, the property's storage location (if any), and borrowed references to get/set.

With the Proxy getOwnPropertyDescriptor call the borrowed get/set reference is a problem because the trap result is quite likely not reachable once it is popped (as expected by the current internal call sites).

So, for now, when getOwnPropertyDescriptor trap is invoked, I'm just checking and filling in only the property attributes and the value, and nothing else. This is enough for enumeration but not for other call sites.