Add several useful blueprint modding functions

[lL1l1]

Description of the proposed changes

• Add a function that allows creating hot-reloadable blueprint modding functions, in contrast to hooking ModBlueprints which requires a full session restart.
• Using "__nil" in a merge blueprint value sets the field to nil. This greatly simplifies anything involving such bps.
  • This is an alternative solution to the issues in Improve merge bp compatibility with removing fields #6637
• Document the old way of adding/removing categories from blueprints.
• Add a function to add categories to a blueprint.
• Add a function to safely merge changes into weapon blueprints (since bp.Weapon is an array).
  • Add a way to do this from inside a merge blueprint. I added this since such a way exists for categories too. It's different from the function in that it's easier to understand is more contained, while the function can be applied programmatically.

Testing done on the proposed changes

Using the debug blueprint file works as expected.

Additional context

These are useful things I made while working on the SACU rebalance mod.

Checklist

• Changes are annotated, including comments where useful
• Changes are documented in the changelog for the next game version