no-unused-prop-types: change reported node to property key

[golopot]

The rule no-unused-prop-types currently reports the property value of the unused propType. I think it is more appropriate to report the property key.

// current behavior
Foo.propTypes = {
  neverUsed: PropType.number,
//           ~~~~~~~~~~~~~~~
// PropType is defined but never used. 
}

// proposed behavior
Foo.propTypes = {
  neverUsed: PropType.number,
//~~~~~~~~~
// PropType is defined but never used. 
}

[ljharb]

Sounds like a great improvement.