Rename category -> itemGroup

[svr333]

Description

This is a follow up to #3139, a lot of comments and variables and error messages needed to be updated still.
This is what that does

Two more follow up questions:

• in wiki.json, Christmas-Seasonal-Category is mentioned, change to Christmas-Seasonal-ItemGroup?
• do I update messages.yml (and other files) keys? (eg: guide.tooltips.open-category -> guide.tooltips.open-itemgroup)

Checklist

• I have fully tested the proposed changes and promise that they will not break everything into chaos.
• I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• I followed the existing code standards and didn't mess up the formatting.
• I did my best to add documentation to any public classes or methods I added.
• I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• I added sufficient Unit Tests to cover my code.

[TheBusyBiscuit]

Looks good, just one small mistake.

Thanks!

[svr333]

Also what about these @TheBusyBiscuit

• in wiki.json, Christmas-Seasonal-Category is mentioned, change to Christmas-Seasonal-ItemGroup?
• do I update messages.yml (and other files) keys? (eg: guide.tooltips.open-category -> guide.tooltips.open-itemgroup)

[TheBusyBiscuit]

Also what about these @TheBusyBiscuit

• in wiki.json, Christmas-Seasonal-Category is mentioned, change to Christmas-Seasonal-ItemGroup?
• do I update messages.yml (and other files) keys? (eg: guide.tooltips.open-category -> guide.tooltips.open-itemgroup)

Regarding the wiki reference, the wiki page itself would need to be renamed first (if thats wanted).
Message strings... well, they could be updated but they would need to be updated in every class and every locale file. If you wanna go through all of that, sure. Otherwise we can just leave it as is or do it later.

[svr333]

I think I'm going to keep the Christmas seasonal category as-is, because I think the in-game text also still says category over itemgroup, and in this case it does make more sense
its a christmas category

Ill do the other changes in a bit

[WalshyDev]

God GitHub hates large PRs but I couldn't find any typos or anything so looks good