fix: CoachMarkContext and SlottedContextValue types for autocomplete

[unional]

Closes #9422

Note that this PR fix the type which is technically breaking, as it makes the type validation function properly.

The change to CoachMark.tsx shows the breakage:

- export const CoachMarkContext = createContext<ContextValue<CoachMarkProps, HTMLElement>>({});
+ export const CoachMarkContext = createContext<ContextValue<Partial<CoachMarkProps>, HTMLElement>>({});

Although the breakage can be "avoided" by:

- export type SlottedContextValue<T> = SlottedValue<T> & T | null | undefined;
+ export type SlottedContextValue<T> = SlottedValue<T> & Partial<T> | null | undefined;

Keeping the Partial<T> in user land is more flexible and more correct, e.g. if the user want to NOT doing Partial<T> for their specific use case.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Run yarn check-types

[snowystinger]

Thanks, the CoachMark one looks like it's now being handled the same way as the Button in S2, so I think that's fine.