Fix bugs in WebView2 cursor updating

[krschau]

This change updates three areas:

• There was a bug where if a user clicked in the webview, the cursor would no longer update. For example, when moved over a button, we expect the cursor to change from an arrow to a hand. This was because on pointer up, we release pointer capture. Doing so would change m_isPointerOver to false, and we would no longer apply cursor updates in UpdateCoreWindowCursor. By moving that logic to HandlePointerExited, we get the expected behavior.
• This change also removes m_requestedCursor. Instead of caching the cursor, we get it from the core controller in UpdateCoreWindowCursor. In addition to not needing to cache it, this also means we won't set the cursor to null the first time the pointer enters the webview.
• Lastly, we used to cache which button was pressed on PointerPressed, and expected that to be the button released on PointerReleased. By getting the PointerUpdateKind at the time of press or release, we don't have to cache the button, and we don't have an instance where one button is released but we send a message with a different button to the core webview. If multiple buttons are pressed or released, we only get the first PointerPressed or last PointerReleased message, so we don't have to worry about releasing pointer capture while another button still has capture.

https://task.ms/40563051
https://task.ms/40563093

How Has This Been Tested?

Verified manually. Added CursorClickUpdateTest in addition to CursorUpdateTest.

[krschau]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[ghost]

🎉Microsoft.UI.Xaml v2.8.1 has been released which incorporates this pull request.:tada:

Handy links:

• Release Notes
• Official Nuget Release