[CPP] Fix manual sorting doesn't stack items

[ampitere]

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• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
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• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

When manually sorting stackable items it does not stack the items as expected. To fix this we'll expand upon SmallPacket0x029 (item movement) and stacking logic similar to SmallPacket0x03A (sort inventory). Resolves #4929

Steps to test these changes

1. Turn off auto sorting (Config > Gameplay > Inventory > Sort)
2. !additem gold_beastcoin 1
3. !additem gold_beastcoin 2
4. !additem gold_beastcoin 12
5. Attempt to manually sort the small stacks together, they should combine
6. Attempt to manually sort the combined small stack with the large stack, it should move but not stack over max stack size (12)