Change find() to return the same index type as pairs() (#24774)
This does not change anything for AbstractVector, Tuple and general iterables,
which continue to use linear indices. For other AbstractArrays, return
CartesianIndexes (rather than linear indices). For AbstractDict, AbstractString
and NamedTuple, return keys (previously not supported at all).
Relying on collect() to choose the return element type allows supporting
any definition of pairs(), including that for Dict, which creates a standard
Generator for which eltype() returns Any.
