core: Set loader to cancelled if target clip is avm1_removed

[ChrisCPI]

Description

Fixes #23402

When calling movie_loader and movie_loader_data, it will check if the target clip has been avm1_removed(). If so, then it will set that loader's status to Cancelled and return early, preventing the loaded movie from executing.

Generally, movie_loader will detect if the clip was removed when it was removed immediately after trying to load a movie into it. Otherwise, movie_loader_data will detect it if the clip was removed in-between the time when the load started and when it finished.

Testing

3 new tests have been added, but only avm1/load_cancel_via_removemovieclip is passing. There is likely some underlying issue with unloadMovie and MovieClipLoader.unloadClip that is causing the other two to fail.

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.