Stdlib: exception previous-chain compliance guard (#5104)

[PurHur]

Summary

• Closes Stdlib: Exception::getPrevious() / Throwable chain — missing VM method (zend_exceptions.c) #5104 — Exception three-arg ctor + getPrevious() + __toString chain already green on master (Stdlib: Throwable getTrace()/getTraceAsString()/__toString() — VM introspection missing (zend_exceptions.c) #7159 landed the Throwable introspection surface).
• Adds test/repro/exception_previous_chain.php maintainer repro and test/compliance/cases/stdlib/exception_previous_chain.phpt VM guard.

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php test/repro/exception_previous_chain.php
php vendor/bin/phpunit test/compliance/VMTest.php --filter exception_previous_chain'

previous_is_exception=true
previous_message=inner
inner_previous_null=true
chain_has_next=true
chain_has_inner=true
chain_has_outer=true
OK (1 test, 1 assertion)

Closes #5104

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