Language: reject $this reassignment at compile time (#4865)

[PurHur]

Summary

• Reject assignment to $this in user methods at compile time with Zend message Cannot re-assign $this (lib/Compiler.php, zend_compile.c parity).
• Belt-and-suspenders VM guard on TYPE_ASSIGN, TYPE_ASSIGN_REF, and inc/dec write slots.
• Repro, compliance phpt, and unit test.

Closes #4865

php-src reference

• Zend/zend_compile.c — assignment to $this rejected at compile time

Verification

Issue repro:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro-maintainer/parity_reassign_this.php; echo zend:$?
php bin/vm.php test/repro-maintainer/parity_reassign_this.php; echo vm:$?
'

PHP Fatal error:  Cannot re-assign $this in /compiler/test/repro-maintainer/parity_reassign_this.php on line 5
zend:255
parseAndCompile failure: target=/compiler/test/repro-maintainer/parity_reassign_this.php: Cannot re-assign $this
PHP Fatal error:  Cannot re-assign $this in /compiler/lib/Compiler.php on line 288
vm:255

Targeted tests:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
vendor/bin/phpunit --filter ReassignThisCompileTest
vendor/bin/phpunit --filter reassign_this
'

OK (2 tests, 3 assertions)
OK (2 tests, 2 assertions)

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