made show fallback optional

[maccesch]

Fixes #1760

[gbj]

Dare I mention that the same Fallback type should be reusable for Suspense and Transition too, unless I'm missing something? Would you like to try to include that in this PR? If not, we can handle it separately.

This seems obviously good to me, I don't know why I didn't do it a long time ago other than stubbornness and lack of creativity! 😄

[maccesch]

Working on a project for a long time seems to make some amount of tunnel vision unavoidable. At least for me it's like that.

I renamed the struct to ViewFn because that seems to describe it better to me. I didn't add this to ErrorBoundary because I don't think we should encourage irgnoring errors 😄 .

[gbj]

Working on a project for a long time seems to make some amount of tunnel vision unavoidable.

This is 100% my experience too!

Thanks so much for taking the time — this will be such a nice quality-of-life improvement for everyone.