Resolves to the CngxValueTransformer a control publishes for translating
between its typed value and its DOM display string:
format(raw) - on value-from-outside writes (field, setValue, value.set)
parse(display) - on user-input and blur reads
Provided on the control's own host by the directives that intercept its
value channel: CngxInputMask, CngxInputFormat, CngxNumericInput. A host
that needs the typed-to-display mapping resolves it here.
Optional and orthogonal to CNGX_FORM_FIELD_CONTROL: that token declares
"I am the focusable element", this one "I translate its value channel" - one
directive may provide both.