[Future]: Express what is below as a type constraint.

[todo]

LDfields/packages/generic-field-base/src/index.ts

Lines 9 to 14 in e517e19

	// TODO [Future]: Express what is below as a type constraint.
	// GenericField<Rendered, Props, ExtraData> should be the signature
	// of the constructor.
	// implements GenericField<Rendered, Props, ExtraData>
	constructor(public modifier: keyof Props & string) {
	super();

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This issue was generated by todo based on a TODO comment in e517e19. It's been assigned to @jeswr because they committed the code.