Support ES5 this handling in strict mode

[p-bakker]

While Rhino has some support for strict mode, a large chunk is missing, namely the handling of the this value in functions in strict mode.

Basic gist:

• if there's no explicit this value, don't default to globalThis
• if the this value is a primitive, don't box it as an Object

Implementing this one has its challenges, mainly the fact that the Callable interface that is used throughout the Rhino codebase and by 3rd party code that integrated/interacts with Rhino assumes this is an instance of ScriptableObject, a assumption that no longer holds true.

See #442 (comment) for some details

[p-bakker]

Related issues: #442, #564, #639, #173

[p-bakker]

PR #39 attempted to fix this as well (resulting in breaking public API), see (amongst others) the following commits:

• b4b6d70
• 796fa2b
• d9c1d49
• 0509ae4