Convert `eltype` using specific types

The conversion to `float` works, so ideally, conversions to specific element types should work as well:
```julia
julia> float(0..1)
0.0..1.0

julia> Float64(0..1)
ERROR: MethodError: no method matching Float64(::IntervalSets.ClosedInterval{Int64})

Closest candidates are:
(::Type{T})(::AbstractChar) where T<:Union{AbstractChar, Number}
@ Base char.jl:50
(::Type{T})(::Base.TwicePrecision) where T<:Number
@ Base twiceprecision.jl:266
(::Type{T})(::Complex) where T<:Real
@ Base complex.jl:44
...

Stacktrace:
[1] top-level scope
@ REPL[67]:1
```