Add an option for flat reductions to damage multipliers

[kgscialdone]

As discussed on Discord, see feature request.

Adds a Reduction type to Damage Multipliers, which reveals a new Reduction Amount field when selected. When damage is applied, relevant reduction amounts are subtracted from the damage after multipliers, and cannot reduce damage below 0.

Potential good features to add before merging:

• Before/after multipliers option (5e RAW is always after iirc but homebrew/DMs may differ)
• Adjustable minimum value after reduction (somewhat tricky since different reductions may have different minimums)

Potential gotcha: the value of a multiplier in Reduction mode is -1, which may cause issues with calculations that naively assume the value can be multiplied in directly. It may be best to adjust this before merging, but I'm not sure what the best alternative would be.

[kgscialdone]

Okay, I think I've finally got all the major kinks worked out of this. Should be properly ready for review and potential merge.

[CLAassistant]

All committers have signed the CLA.

[ThaumRystra]

I will update this to the 2.1 action/computation engine changes and merge once that's done.

[ThaumRystra]

update to 2.1