chore(react-utilities): improve Slot signature to be more presentable in Storybook

[bsunderhus]

Previous Behavior

Slot signature gets broken into its type equivalence by TS compiler (more info #27105 (comment)):

New Behavior

Modifies Slot signature to be more presentable on Storybook's docgen extraction.

Problems involved with this Solution

We verify if the first generic provided to Slot<T> is a singleton value (a value that is not an union of other values). This PR modifies this signature to be internal to another type (SlotProps), as this type verification is one of the causes of why Slot signature ends up being processed by TS compiler the way it does.

Previously if someone would provide a union we'd have:

By moving the singleton verification down to another type (SlotProps) we'll be allowing some types in situations they should probably not be allowed, in this case in particular is not so problematic as we're only allowing null:

Related Issue(s)

• Fixes [docs]: improve Storybook prop table for Slot<T> type #27105

[fabricteam]

Perf Analysis (@fluentui/react-components)

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
FluentProviderWithTheme	virtual-rerender-with-unmount	80	67	10	Possible regression

All results

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
Avatar	mount	609	623	5000
Button	mount	297	297	5000
Field	mount	1152	1126	5000
FluentProvider	mount	719	708	5000
FluentProviderWithTheme	mount	91	88	10
FluentProviderWithTheme	virtual-rerender	34	33	10
FluentProviderWithTheme	virtual-rerender-with-unmount	80	67	10	Possible regression
MakeStyles	mount	845	846	50000
Persona	mount	1728	1747	5000
SpinButton	mount	1412	1363	5000
SwatchPicker	mount	1524	1505	5000

[fabricteam]

📊 Bundle size report

✅ No changes found