Allow users to specify the output eltype of map

[MasonProtter]

The way map currently works is that when you map over an Observable, the eltype of the new Observable is determined by the first application of the mapping function, i.e.

julia> using Observables

julia> o1 = Observable{Any}(true);

julia> o2 = map(o1) do x
           @info "Updating o2 from o1" x
           x + 1
       end
┌ Info: Updating o2 from o1
└   x = true
Observable(2)

julia> eltype(o2) # This is bad!
Int64

julia> o1[] = 1.5 # because of this
┌ Info: Updating o2 from o1
└   x = 1.5
ERROR: InexactError: Int64(2.5)
Stacktrace:
  [1] Int64
    @ ./float.jl:994 [inlined]
  [2] convert(::Type{Int64}, x::Float64)
    @ Base ./number.jl:7
  [3] setproperty!(x::Observable{Int64}, f::Symbol, v::Float64)
    @ Base ./Base.jl:52
[...]

This PR keeps the current default behaviour, but adds a kwarg with two new options out_type = :infer

julia> o2 = map(o1; out_type=:infer) do x
           @info "Updating o2 from o1" x
           x + 1
       end
┌ Info: Updating o2 from o1
└   x = true
Observable{Any}(2)


julia> o1[] = 1.5
┌ Info: Updating o2 from o1
└   x = 1.5
1.5

julia> o2[]
2.5

and out_type::Type:

julia> o3 = map(o2; out_type=Number) do x
           x
       end
Observable{Number}(2.5)

[MasonProtter]

I had to weaken these tests because the printing of this sort of thing has changed on v1.11, and it was causing the tests to fail.

[bvdmitri]

Perhaps also look at https://github.com/ReactiveBayes/Rocket.jl which already provides this functionality, see here

[SimonDanisch]

So, I think we specifically had:

o1 = Observable{Any}(true);
 o2 = map!(Observable{Any}(), o1) do x
     @info "Updating o2 from o1" x
     x + 1
 end

For this pattern, which I think is nicer than a keyword argument?

[MasonProtter]

I find that pattern a lot more annoying to write than using a keyword personally, and second of all, that pattern doesn't cover the :infer keyword I added.