Fix double exception raise in Proxy [[Delete]] invariant checks

[SAY-5]

ecma_proxy_object_delete_property runs the two ProxyDeleteProperty result invariants (non-configurable property, non-extensible target) in sequence. When a deleteProperty trap returns true for a non-configurable property on a non-extensible target, both checks fail and ecma_raise_type_error is called twice. The second call raises a TypeError while one is already pending, which trips the !jcontext_has_pending_exception () assertion and aborts assertion-enabled builds; release builds overwrite the pending exception state. Per the spec the non-configurable check throws and returns, so the extensibility check should only run when the property is configurable. Added a regression case to tests/jerry/proxy_delete.js that aborts before the change and passes after.

Fixes #5296