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?

[zithith]

OK, realised that we couldn't take the same approach as the damage mods this time, for a number of reasons, but the most obvious being that to do so would have been a complete refactor of the roll engine.
Instead I've given a couple of fields (one in the item details of activatable items with skill tests for global mods, and one in the roll configuration pop-ups for temporary ones) that allow the users to enter formula in a similar way to the damage override, only this will be an extension of what is already there. They'll still set the skill and attribute to use for the item's skill rolls as before and then this gets tacked on the end.

There is more to say on skill/attribute selection however as I allowed for skill ranks without an attribute, perhaps we should also allow attribute and no skill ranks? (Then the user can get full control of their roll formula, but also get some level of assisted setup)

But this bit is still a bit messy actually. Worth getting input and then sorting out alongside the sheets brush up in 0.3