GroupHeader: tighten typings

[natalieethell]

Pull request checklist

• Addresses an existing issue: Fixes #0000
• Include a change request file using $ npm run change

Description of changes

Tightens typings in GroupHeader, which is used in GroupedList. Right now, I'm pretty sure the prop isGroupLoading will never work. headerProps doesn't exist on IGroupHeaderProps, so the isGroupLoading const would have always been undefined.

The original IGroupHeaderProps had headerProps, but when that API was changed, this part of the code probably went unnoticed since it didn't have proper typing.

Focus areas to test

Ensure that IGroupHeaderProps's isGroupLoading (extended from IGroupDividerProps) works correctly now.

I'll see if I can write a test for this change, too.

Microsoft Reviewers: Open in CodeFlow

[size-auditor]

Bundle test	Size (minified)	Diff from master
GroupedList	118.86 kB	-19 bytes
ShimmeredDetailsList	219.625 kB	-19 bytes
DetailsList	208.801 kB	-19 bytes

  Exceeds Tolerance       Exceeds Baseline       Below Baseline     1 kB = 1000 bytes

[msft-github-bot]

🎉[email protected] has been released which incorporates this pull request.:tada:

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