Support more spec-compliant ScriptableObject extensions

[gbrail]

This fixes a problem in which ScriptableObject subclasses couldn't support some of the newer code, like Object.assign, because it depends on internal implementation details of ScriptableObject. This changes two things:

• Adds new overridable methods to ScriptableObject called "putOwnProperty".
• Call those in internal places instead of calling "putImpl" directly.
• Catch exceptions in Object.assign if "getAttributes" throws an exception, as the default implementation does.

This makes it possible to address the problem identified in #1558.

To be fully compatible, a ScriptableObject subclass that wants to add its own properties should override "putOwnProperty" INSTEAD of overriding "put" and delegate to the superclass for any unknown properties.

Added a new test that demonstrates this technique.

I'd be interested in feedback here -- this is not a fix to all spec-compliance problems but it's not the ugliest fix in the world either.

[rbri]

ok for me

do you have an idea why this does not fix getOwnPropertyDescriptor/trap-is-null-target-is-proxy.js
while it fixes all the others especially getOwnPropertyDescriptor/trap-is-undefined-target-is-proxy.js ?

[gbrail]

Something like this might also be a path forward for getting strict mode right -- we can keep Scriptable, but add a new interface for objects that want to extend in a more spec-compliant way, and have code in ScriptableObject and ScriptRuntime adapt if the new interface is present.

[gbrail]

Also, as for the tests that were fixed -- this might be because NativeProxy doesn't implement "getAttributes". Arguably it should, or we should change the contract between the runtime and ScriptableObject extensions more.