[#1933] Fix issues with default skills not using alternate ability defined in config

[arbron]

This removes the default values defined for abilities and skills in template.json and modifies _prepareBaseAbilities and _prepareBaseSkills methods to get their data structures from the ability & skill schema fields rather than from the template.

To accomplish this I have implemented the dnd5e.utils.getSchemaField method that transverses the provided key path to get the DataField for a specific location within a specific DataModel. This method will also be useful later for instances where we want to use information from our data models for tasks like applying active effects or preparing specific data.

A few changes also had to be made to AbilityConfig, SkillConfig, and ActorSheet5e#_onCycleSkillProficiency to handle skills that don't yet exists in the source data.

Blocked by https://github.com/foundryvtt/foundryvtt/issues/8813 Resolves #1933