[Homebrew] Add Critical Hit Modifier

Currently class features that modify critical hit chance are limited to text descriptions, however with the new dice roller able to automatically detect a critical (and display special effects) it makes sense to add a modifier to Homebrew that enables the critical hit chance to be set explicitly.

There are two main features that require this that I'm aware of:

• Champion: The Champion sub-class of Fighter has Improved Critical and Superior Critical, which improves the chance of a critical hit on their weapon attacks by 1 (or sets to 19 and then 18).
• Gunslinger: The Critical Role Gunslinger sub-class of Fighter has a similar ability in Vicious Intent, which improves the chance of a critical hit for firearm weapon attacks by (or sets to 19).

There may be others however. To cover these two, it might make sense to add a new Critical Hit Bonus modifier, with sub-types for different kinds of weapon (as well as one for all weapons) where the fixed number acts as a reduction to critical hit chance, so a value of 1 reduces from 20 to 19 or 20, 2 would reduce to 18, 19 or 20 and so-on.

This could then be used by the dice roller, Avrae and so-on to automatically apply critical hits correctly.

There is also another set of cases whenever we gain the ability to set temporary effects, such as the Hexblade's Hexblade's Curse ability.