Translation inheritance

[bartfeenstra]

Allow locales to inherit from others. That way, en-GB can inherit from en-US, override the few translations from en-US it needs to, and translations are still reported as 100%.

Proposals

• Allow a metadata file to be place in each locale's directory
• a list of zero or more fallback locales can be provided, possibly as a regex pattern

NOTE: we cannot put a manifest file in an assets directory because those can be overridden? Or would we merge them? Should we do this in project config instead?