`isassigned` missing for a `AbstractQuasiArray`

Due to changes to `copyto!` for arrays on julia v1.11 that now check if an index is assigned, it would be good to have `isassigned` defined for an `AbstractQuasiArray`. This should get tests passing when e.g. a `SubArray` is being materialized.

Failure: https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_date/2024-04/23/ContinuumArrays.primary.log