StepRangeLen broken for AnchoredIntervals on Julia 1.8

[BaileyBox]

StepRange has added the ability to also use Length as a parameter in Julia1.8 (along with making stop and length as only constructors possible). This change somehow broke StepRangeLen with AnchoredIntervals.

An example showing how this is broken:

Julia1.6:

using Intervals

s = AnchoredInterval{-1}(1):1:AnchoredInterval{-1}(5)
r = s[1:5]

s isa StepRangeLen # false
s isa StepRange # true
r isa StepRangeLen # true
r isa StepRange # false

Julia1.8:

using Intervals

s = AnchoredInterval{-1}(1):1:AnchoredInterval{-1}(5)
r = s[1:5]

s isa StepRangeLen # false
s isa StepRange # true
r isa StepRangeLen # false
r isa StepRange # true

This failure can also be observed in the CI and Nightly pipelines. Current test failing is at anchoredinterval.jl:586.