swap stable feature with nightly and empty feature set testing

[remkop22]

@gbj I based this branch of your previous work here #571.

I found out by some trial and error that specifying an empty set [] in skip_feature_sets actually skips the empty set 🤯.

I also added a separate test that tests all non-doc-tests with an empty feature set on a stable compiler. This should at least verify that Leptos works as it should on stable without additional feature flags.

Also added feature flags to the integration libraries (@gbj If you have reasons to dislike this due to breakage I'll happily remove it).

[gbj]

Thanks — This is great to know, and seems really helpful. One thing that would also be good would be to remove "csr" from the default features for leptos, so that people need to opt in to CSR and not opt out. This would mean people don't automatically do default-features = false for SSR and then have to opt back in to nightly.

I'm happy to schedule this for some upcoming 0.3 when we're reading to make breaking changes.

[gbj]

I started this over in #1227, and just migrtaed all the tests over to work with stable.