Extras as local-ranged requirement targets
Add support for extras as requirement targets (as already exists for e.g. improvements and unit types), with associated local-ranged requirements (RoadFlag, Extra, ExtraFlag).
This is possible now since the "exists on tile" kind of requirement has been moved to the Tile range with hrm #695469. However, to avoid potential rscompat issues, adding new things to the Local range should not be done until after #43708.
Probably most sensible to delay this until after #43809, to make adding the new requirement target easier.
A few notes on this patch: