implement `ProjectTo(::NamedTuple)`

[mzgubic]

Needed in JuliaDiff/ChainRulesTestUtils.jl#224 (see comment)

Simplest possible implementation.

[devmotion]

Can we just add the suggested implementation for now? Or are we aware of any problems?

If not, I could put together a PR later today.

[mzgubic]

I am not aware of any but haven't thought about this very deeply.