avm2: Implement XML.attribute(s) using get_property_local

[evilpie]

Description

The specification and Flash both implement it this way.

See also #24330

Testing

Should be covered by existing tests.

Checklist

• I, a human, have self-reviewed this PR and fully understand the changes within.
• I have made or updated tests where possible.
• All of my commits are properly scoped, compile successfully, and pass all tests.
• This PR does not make sense to split up into smaller PRs.
• An LLM was involved in the authoring of this code.