BREAKING CHANGES
Eta no longer supports using @include and includeFile because our implementation added bundle size and complexity and was inflexible (ex. await @include did not work).
I decided not to release a new breaking version, since to my knowledge no projects are using Eta yet. If you do need to transition, change <% @include(...) to <%~ E.include(...) %> (think E for "Eta"), and the same for includeFile.