Make RuntimeGuard public

[ealmloff]

This is a different approach to #1441 that just makes the runtime guard public. This API is slightly more complex because it uses drop handlers, but can be more flexible

Closes #1465

[jkelleyrtp]

I don't see a drop handler here?

[ealmloff]

I don't see a drop handler here?

That isn't added in this PR. This PR just makes it public. In

dioxus/packages/core/src/runtime.rs

Line 157 in 8b411b8

pop_runtime();

:

impl Drop for RuntimeGuard {
    fn drop(&mut self) {
        pop_runtime();
    }
}