[ENHANCEMENT] Define `SparseArraysBase.OneElement`/`SparseArraysBase.oneelement`

[mtfishman]

Define a sparse array type with only one nonzero value, analogous to ITensors.onehot and FillArrays.OneElement.

The plan plan will be for ITensors.onehot to get deprecated in favor of SparseArraysBase.oneelement, which will be a named dims array wrapping a sparse SparseArraysBase.OneElement.

[mtfishman]

Closed by #26.