fix(core): disallow transitions to same node (resolve #900)

[epaminond]

No description provided.

[dmk23]

@epaminond Why should this be disallowed rather than fixed?

There is an obvious use case for this - any user input that must get validated and might be re-tried

The usage workaround (of adding separate nodes for input capture vs. validation) makes flow diagrams clunkier... I'd much rather prefer to have fewer nodes with stronger functionality

I suppose the real fix should involve some way to kick the dialog back to onReceive?

[epaminond]

@dmk23 ,

1. I can see this is actually disallowed
2. This transition looks ugly and you won't be able to select/delete it from the UI
3. This way it's easy to cause infinite loop

[dmk23]

1. That's why validating/retrying user input is a problem now
2. Most transitions (and node boxes) look ugly now, Flow UX will need major improvements in any case (yes, I have gotten this exact feedback showing Botpress UX to potential non-technical users)
3. Indeed, and that's why there should be clear warnings. Also, same kind of loops are possible with any workarounds (e.g. using 2 nodes for this kind of validation workflows)

[epaminond]

Merging this for now to keep things consistent. At a later point we may always return and implement self-pointing feature.