Add always

[davidkpiano]

This PR adds support for the always transition, which works exactly the same as eventless transitions:

on: {
- '': 'someTarget'
},
+always: 'someTarget'

As the tests show, this is not a breaking change. The original way for defining eventless transitions will continue to work.

TODO:

• Docs
• Tests

[changeset-bot]

🦋 Changeset is good to go

Latest commit: 72d2eef

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Name	Type
xstate	Minor
@xstate/analytics	Major
@xstate/graph	Major
@xstate/immer	Major
@xstate/react	Major
@xstate/test	Major
@xstate/vue	Major

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[Andarist]

Should we introduce a deprecation warning for '' right away?

[davidkpiano]

Going to merge this in (all comments resolved) - won't release yet, in case a critical change is needed. @Andarist

[wildskyf]

we should also update the docs here:
https://xstate.js.org/docs/guides/events.html#null-events

[davidkpiano]

we should also update the docs here:
xstate.js.org/docs/guides/events.html#null-events

Feel free to make a PR for this, or an issue please