Provide diff tokens for nodes

Attached is a patch against 8.x-1.x-dev.

Found an issue with the bubbling of cache metadata, so new patch and interdiff attached. Also updated the tests to assert correct metadata.

Like the previous patch, this requires PHP 7.4 or later, but I think this is just due to some of the type hinting. Looks like the default testing is currently for PHP 7.3. If needed, I can probably provide an updated patch that is compatible with 7.3 as well. Let me know.

Thanks @acbramley! I created an MR against the 2.x branch, that should be all that was missing to meet the contribution guidelines for NR.

One small note on this part of the code in TokenProvider:

      $default_layout = $this->diffLayoutManager->getDefaultLayout();
      if ($default_layout === '') {
        $replacements[$original] = $this->t('(No default diff layout configured.)');
        continue;
      }

This was an empty() check in the original patch for 1.x, because I actually ran into this during development. empty() checks are no longer allowed by the phpstan configuration of this module. Thus I looked at this again and the method has now a string return type hint. So I changed this to check for an empty string instead. But: getDefaultLayout() still has this return statement return \reset($plugins); and if $plugins is an empty array, it will not return a string, but boolean false. This means that we could probably remove this check in TokenProvider entirely, since the Layout Manager will then generate a white page (wrong return type). Or we could fix the wrong return type. For now I didn't change anything about this in the layout manager, because unlike the other change in the layout manager that is a part of this patch, this bug fix is not needed to pass tests here, so it could be done in a follow-up issue, if desired.

Alright, separate module is not gonna happen, so let's close this won't fix.