feat: Allow plugins to declare an action invalid

[delucis]

Closes #962

This adds support for an isInvalid method in boardgame.io plugins. Similar to the current noClient plugin method, this allows the reducer to ask plugins if everything is OK or if the current event/move is invalid from their point of view. If one or more plugins returns a truthy value from its isInvalid method, the reducer will fall back to its initial state, analogous to what it does when a move returns INVALID_MOVE.

Currently this is not in use but the intention would be to use it for the events plugin, following on from #957. It also allows for other custom use cases, like state validators.

Notes

• isInvalid should return a string if the action is invalid. This could be less strict if that seems desirable.

• The error returned to the client is of the form

  {
    type: 'action/plugin_invalid',
    payload: {
      plugin: 'name of plugin that declared action invalid',
      message: 'string returned by plugin’s isInvalid method'
    },
  }

• If more than one plugin’s isInvalid method returns a string, we just error with the first of them, rather than reporting all of them.

[vdfdev]

10/10 very readable and well tested code, thank you!

[delucis]

Thanks for the review @vdfdev!