This simplifies some logic in `languagetree.lua`, removing the need for
`_has_regions`, and removing side effects in `:included_regions()`.
Before:
- Edit is made which sets `_regions = nil`
- Upon the next call to `included_regions()` (usually right after we
marked `_regions` as `nil` due to an `_iter_regions()` call), if
`_regions` is nil, we repopulate the table (as long as the tree
actually has regions)
After:
- Edit is made which resets `_regions` if it exists
- `included_regions()` no longer needs to perform this logic itself, and
also no longer needs to read a `_has_regions` variable
02ea0e77a1