Registers application-wide defaults for every CngxFormField in scope.
Each with* feature contributes one slice of FormFieldConfig; features
apply left to right, so a later feature overrides an earlier one on the
same key.
Returns EnvironmentProviders, so it sits at an environment injector -
bootstrapApplication's providers, or a lazy route's providers. For a
component sub-tree use provideFormFieldAt. Resolution is nearest-wins and
replace, not merge: a route-level provideFormField shadows the root one
for that subtree rather than deep-merging into it.