Set AllowDynamicProperties on TypedElement and UnaddressableTypedElement

[Krinkle]

Fixes #4776.

[Daimona]

This won't work in PHP 8.0 and 8.1, which are recent enough to support attributes, but old enough that they don't have the AllowDynamicProperties attribute. Easiest thing would be to create a stub for the AllowDynamicProperties attribute class that would only be loaded in those PHP versions. I believe .phan/stubs would be the right place for it, but I'm not familiar with the configuration used for phan's own tests.

[Daimona]

Also, just because I had second thoughts: the AllowDynamicProperties class shim does not need to be exported for use outside of phan's CI. PHP does not require that the classes used in attributes actually exist, and therefore the attribute itself is a no-op in PHP < 8.2.

Aaaand also, .phan/stubs seems to be the correct location for this. You can copy the stub from MediaWiki's phan config, just make sure to move the documentation to an actual class doc comment or phan will complain.

[rlerdorf]

Merged manually with some tweaks