Investigate / document `EntityHeartDrop`

EntityHeartDrop appears to be used to drop items other than hearts out of entities and may not be accurately named.

Examples:

• Stone mask from candle table in lib
• Coffin monster vial in cat

It appears as though this entity is not specific to heart drops but is a more general prize drop entity. https://github.com/Xeeynamo/sotn-decomp/blob/master/src/st/e_collect.h#L1471-L1507

Documentation and investigation should be able to also find a better name and how it differs from the existing EntityPrizeDrop https://github.com/Xeeynamo/sotn-decomp/blob/master/src/st/e_collect.h#L333