Improved accuracy of IconProps interface

[amcclain]

• Use the IconName and IconPrefix types provided by FontAwesome.
• Define a ResolvedIconProps interface, for specific usages where we know iconName and prefix have been applied and will be present.

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• Caught up with develop branch as of last change.
• Added CHANGELOG entry, or determined not required.
• Reviewed for breaking changes, added breaking-change label + CHANGELOG if so.
• Updated doc comments / prop-types, or determined not required.
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[amcclain]

I started looking at this due to warnings being thrown around icons that turned out were actually caused by 9435122

But I had started tightening things up and still there is some utility in taking these changes. My only concern is that the use of the IconName exported type from FA might flag false-positive warnings of some kind, or require apps that are passing icon names as vars to more precisely type them. Not sure that would happen in practice though, or be a blocker - could be helpful...