Use types to express the above condition

[todo]

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

Lines 35 to 40 in e517e19

	* TODO: Use types to express the above condition
	*/
	targets?: (keyof Props & string)[] | undefined;
	get fieldTargets(): (keyof Props & string)[] {
	return this.targets ?? this.modifies;

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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.