Convert `FlashObject` to native class

[gilest]

This will be breaking due to

• Public type declarations
• The getFlashObject public API

To do

• [x] Upgrade guide -> You cannot call .get() etc on FlashObject instances
• [x] Update type declarations

As for these two I'm not sure what their purpose is, I couldn't find any mention of them in the docs or PRs/issues. I don't think they are public API...

• Document on('didDestroyMessage') event -> onDidDestroyMessage callback
• Document on('didExitMessage') event -> onDidExitMessage callback