Lazy concatenation of arrays

Is your feature request related to a problem? Please describe.
Concatenating xarray objects forces the data to load. I recently learned about this object allowing lazy indexing into an DataArrays/sets without using dask. Concatenation along a single dimension is the inverse operation of slicing, so it seems natural to also support it. Also, concatenating along dimensions (e.g. "run"/"simulation"/"ensemble") can be a common merging workflow.

Describe the solution you'd like

xr.concat([a, b], dim=...) does not load any data in a or b.

Describe alternatives you've considered
One could rename the variables in a and b to allow them to be merged (e.g. a['air_temperature'] -> "air_temperature_a"), but it's more natural to make a new dimension.

Additional context

This is useful when not using dask for performance reasons (e.g. using another parallelism engine like Apache Beam).

Start with xarray/core/indexing.py at the linked lazy-indexing object, then trace the xr.concat([a, b], dim=...) entry point. Done means concatenating along one dimension does not load data from either input, including for non-dask usage.