Keep items expression and effect

[moraedu]

Description

Adds the missing keep inventory effect and expressions.

Marking it as a draft since I still haven't tested it but want opinion on whether I should add a pattern to not clear the drops for the keep items effect, since that's kind of the whole reason why I added the effect in the first place so that users don't have to do it themselves.

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Target Minecraft Versions: 1.13+
Requirements: Paper
Related Issues: N/A

[Fusezion]

Did a minor once through, just some nitpicks and mentions I haven't seen yet

[moraedu]

I am planning to merge EffKeptItems to the EffKeepInventory, I did not realize there was already a keep inventory effect. Also, @Moderocky has pointed out that the syntax of both might need some rewording, so if anyone can come up with different syntaxes for them then I am all ears

[moraedu]

I decided to remove the kept items expression as its behaviour was rather unclear, I could have just renamed it to respawn items or something along these lines however I'd rather have the keep specific items pattern be merged rather than have to deal with a better fitting syntax for the expression.

[Moderocky]

I still think this is a little wobbly in execution but it's ok.

[sovdeeth]

I don't think this is implemented properly.
keep inventory maintains the items in the inventory and still has them in the drops.
keep the dirt maintains 1 dirt in the inventory and removes 1 dirt from the drops.
keep all dirt maintains 1 dirt in the inventory and removes it all from the drops

I'd like to see parity between behavior when keeping inventory and keeping specific items, and I think work needs to be done to handle all better.
I'm also of the opinion that keep dirt should keep ALL dirt, and keep 1 dirt should only keep 1 dirt, but I'm not sure if that's feasible.

[sovdeeth]

Closing due to inactivity.