feat: i18n fallback

[jurgenbelien]

Changes

• improves mocks for i18n tests
• creates an array of fallback languages to cascade through
• adds variables to datocms collection whenever a fragment is using variables
• adds locale and fallback locale variables to graphql queries.
• returns an href based not on the currently active locale, but on the locale of the targeted resource

Associated issue

Relates to, but does not completely solve #271

How to test

Checklist

• I have performed a self-review of my own code
• I have made sure that my PR is easy to review (not too big, includes comments)
• I have made updated relevant documentation files (in project README, docs/, etc)
• I have added a decision log entry if the change affects the architecture or changes a significant technology
• I have notified a reviewer

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