Add flat modifiers to attacks

[LLilliputian]

Is your feature request related to a problem? Please describe.
Sometimes when setting up an attack want to use a flat number instead of a skill

Describe the solution you'd like
Should be an alternative where you can just put in a number to you as the modifier instead of having to select a skill

Additional context
Suggested by @stanvdb on 22/10/2024

Checklist:

• I have searched the existing issues to ensure this feature has not already been requested.

[zithith]

I'm gonna grab this.

I'll try and align it to how we do the damage/graze formula approach now. So anywhere the user sees a formula they know nothing is being done automatically and they will need to define @mod if they want the attribute mod included.
Do we include hint text here? Or maybe do we want this sort of detail to be handled in a more centralised source of knowledge?