Add Ultra Burst trigger and make Ultra Necrozma battle-only

[MiquelRForgeFlow]

Change description

Introduce a new ultra-burst-item form trigger and mark Ultra Necrozma as battle-only, triggered by holding Ultranecrozium Z (like Mega/Primal held-item transformations).

Is distinct from held-item trigger to make it easy to customize, like making it trigger from Dusk and Dawn forms instead of default form.

AI coding assistance disclosure

I only asked AI to add the new ultra-burst-item. I defined it manually in the command line, and AI added it in the code.

Contributor check list

• I have written a description of the contribution and explained its motivation.
• I have written tests for my code changes (if applicable).
• I have read and understood the AI Assisted Contribution guidelines.
• I will own this change in production, and I am prepared to fix any bugs caused by my code change.

[MiquelRForgeFlow]

@Naramsim

[MiquelRForgeFlow]

By the way, the battle-only thing is the only assured fact. I can make that fix a part. And leave the ultra-burst-item vs held-item for discussion.

[jemarq04]

It should probably just be held-item specifying the Ultranecrozium Z I think. Though finding a way to specify the required base form to specify Dawn/Dusk form is something we can consider going forward

[MiquelRForgeFlow]

I can make that fix a part

Done in #1619.

Though finding a way to specify the required base form to specify Dawn/Dusk form is something we can consider going forward

Fine by me, let's think how to do it. Probably a new column in some place. Any suggestion?