New item_battery_custom entity

[Blixibon]

This PR adds a new item_battery_custom entity as an analogue to item_healthkit_custom.

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PR Checklist

• My PR follows all guidelines in the CONTRIBUTING.md file
• My PR targets a develop branch OR targets another branch with a specific goal in mind

[1upD]

Approving because I don't see any issues, but I'm curious - why does item_battery_custom need to be a separate class from item_battery? Couldn't the latter support model replacement?

[Blixibon]

In addition to model replacement, item_battery_custom also supports a custom power amount. That functionality could have been added to item_battery_custom as well, but I thought it would be more appropriate to split it off.