Explode values as keys

This is a useful feature we have been using for a while.

It also includes tests.

Marking as RTBC.

1. I see that there is a method added to the unit test class, but the feature that is provided here isn't actually tested. No test is calling getPluginWithKey().
2. A case needs to be added to Drupal\Tests\tamper\Functional\Plugin\Tamper\ExplodeTest::formDataProvider() to ensure that the new setting gets saved. Existing cases need to be adjusted.
3. The description of the setting could use an example. I had to inspect the code to understand what the setting was doing. Maybe change the description into "If enabled, the keys and values of the array will be the same: @code.". And "@code" will then be the code example, encapsulated in code-tags.

Thank you for the patches and reviews.

It also took me a while to understand what was expected based on the description. It feels like maybe this isn't something the explode plugin needs to be doing? But it seems like there is a valid use case here.

Attaching a patch to include this as an additional plugin rather than introducing anything new into the explode plugin. Patch includes update to the chained tamper test to show them working together.

This approach adds a little bit of complexity for the user who now has to add 2 plugins, but if this functionality is needed when using the explode plugin then it could well be useful in other contexts where we have an array from another source that is not the explode plugin.