Build form plugin for Explode process

Thanks for the code! I've added that the restrict setting gets casted to a boolean, else it ends up as the following in the yml file:

strict: '1'

And I think it's better if it would be instead like this:

strict: true

Because the code example in Drupal\migrate\Plugin\migrate\process\Explode also use the "true" and "false" keywords instead of "1" or "0".

I think that the default settings are good!

I would have been great to have test coverage for this plugin as well, but I see that I didn't finish the issue yet where I also added tests for the existing plugins: 🐛 Failure when trying to add a process plugin after "Static map" Needs work

So I guess tests cannot be easily added right now. So perhaps merge it now, add tests later?