The category of fields of characteristic zero

[ScriptRaccoon]

This PR adds the category Fld₀ of fields of characteristic zero. It provides an example of a balanced category that is not mono-regular; no category in the database has previously witnessed this combination (or its dual).

In fact, several additional new combinations are witnessed. The combination script (cf. #352) reveals the following:

Found 12 unique witnessed combinations by the supplied structures (Fld_0):

Directly witnessed:
- balanced ∧ ¬mono-regular
- epi-regular ∧ ¬mono-regular
- quotient-trivial ∧ ¬cogenerating set
- quotient-trivial ∧ ¬essentially small
- quotient-trivial ∧ ¬extremal cogenerating set
- quotient-trivial ∧ ¬mono-regular

Dually witnessed:
- balanced ∧ ¬epi-regular
- mono-regular ∧ ¬epi-regular
- subobject-trivial ∧ ¬generating set
- subobject-trivial ∧ ¬essentially small
- subobject-trivial ∧ ¬extremal generating set
- subobject-trivial ∧ ¬epi-regular

Moreover, the obvious implication subobject-trivial => well-powered has been added (and hence also its dual).

The number of unwitnessed consistent combinations has decreased from 701 to 687.

This PR contributes to this milestone.