Fix ThemeProvider bug of remounting and losing state

[r100-stack]

Changes

A user pointed out a bug where ThemeProvider used to reset the state of its children when inherit was toggled between undefined and defined. (playground)

Fixes the regression introduced in 3.14.0 by #2153. This PR also now passes the e2e test in iTwin/appui#1133 meant to confirm that this regression is fixed.

---

The reason for the reset of the state was because of the following code:

iTwinUI/packages/itwinui-react/src/core/ThemeProvider/ThemeProvider.tsx

Lines 180 to 182 in b8d5fa2

	<ToastProvider
	inherit={themeProp === 'inherit' && !portalContainerProp}
	>

iTwinUI/packages/itwinui-react/src/core/Toast/Toaster.tsx

Lines 107 to 119 in b8d5fa2

	// Re-use existing ToastProvider if found
	if (React.useContext(ToasterStateContext) && inherit) {
	return children;
	}
	return (
	<ToasterDispatchContext.Provider value={dispatch}>
	<ToasterStateContext.Provider value={toasterState}>
	{children}
	</ToasterStateContext.Provider>
	</ToasterDispatchContext.Provider>
	);
	};

Here, when portalContainer/portalContainerProp changes to undefined, the inherit changes from false to true. As a result, ToastProvider now returned children wrapper with ToasterDispatchContext.Provider and ToasterStateContext.Provider instead of just children. When the UI tree position changes, the state is reset (docs).

The fix is to thus keep the UI tree position of children the same regardless of inherit.

Testing

• Confirm that it works in the playground.
• Confirm that the AppUI e2e test passes. (Will be doing it after the PR since @GerardasB said that if the playground passes, the test will also likely pass)

Not sure why some react-workshop Table test images failed. But since they actually look more correct than the old images (i.e. correct element is focused), I approved them for now.

Docs

Added changeset

[mayank99]

I don't love that we are now introducing two additional contexts (which are arguably unnecessary), but it solves the problem. Good fix 👍