Provide focus fallback behaviour for firstContentElement on Modal

[getdave]

#54590 saw a new value firstContentElement for the focusOnMount prop of Modal. This focuses the first element within the Modal's contents as oppposed to anywhere within the Modal.

Folks suggested that if the focus algorithm finds no focusable elements in the content, it ought to fallback to finding any focusable element within the entire Modal.

There was some disagreement as to how that would be done.

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It's going to be pretty challenging with the current implementation though as it relies on moving the focus ref between elements.

Another option could be to console.warn() if no focusable is found?

A console.warn() is probably better than nothing, but I'd personally like to find a way to fallback more gracefully. We can always rework the implementation if necessary, especially now that we have a separate value for this behaviour.

Originally posted by @ciampo in #54590 (comment)