Fix error when updating Bard configs inside fieldsets

[duncanmcclean]

This pull request attempts to fix an issue when imporing Bard fields where you'd get an error if the Bard field is imported from a fieldset, rather than being directly on the blueprint.

Whenever you import Bard fields, it'll enable nearly all of the available buttons in the toolbar (and create sets for blocks, if you're importing from WP's Gutenberg editor).

Since Statamic's Blueprint::ensureFieldHasConfig() method doesn't support ensuring imported fields, we have to manually find/update the fieldsets.

Fixes #17.
Requires statamic/cms#11096.
Requires statamic/cms#11097.

To Do

• Add tests to cover importing single fields
• Consider extracting the "update field config" logic out of the BardTransformer/Gutenberg classes
• Gutenberg: Fix issue where only one set is added on each "run" of the importer
  • The blueprint/fieldset instance is probably out-of-date, so it only adds the last set. Maybe?
• Refactor fieldset field ensuring to use the methods added in [5.x] Allow ensuring fields in fieldsets cms#11097
• Set minimum version constraint of statamic/cms

[duncanmcclean]

We're going to have failing tests in this PR until both statamic/cms#11096 and statamic/cms#11097 have been merged & tagged.