Reconsider string `findprev` semantics

[Keno]

We have (as described in #49547):

julia> findprev(==('x'), "x", 1)
1

julia> findprev(==('x'), "x", 2)

I don't think this makes much sense. It's also inconsistent with:

julia> findprev("x", "x", 1)
1:1

julia> findprev("x", "x", 2)

julia> findprev("ab", "ab", 2)
1:2

julia> findprev("ab", "ab", 3)
1:2

@KristofferC did some digging and found that I may have introduced this original behavior in 5e57c0b#diff-66843bd6e4b813dc28435a71b7a73a22f5fba6da4912e32eaf6d5f58745f8079R1269 and it has propagated from there since without a second thought. I think this might have been a bug. The condition makes sense for forward search, but I don't think it makes a lot of sense for reverse search. Nevertheless, it's been the behavior for many years, so we should probably decide carefully if and how to replace it.

[jakobnissen]

Also see #40120

[oscardssmith]

for more fun:

julia> findprev(==(0x1), UInt8[1,2], 3)
#nothing
julia> findprev(==(0x1), UInt8[1,2], 2)
1
julia> findprev(==(0x1), [1,2], 3)
ERROR: BoundsError: attempt to access 2-element Vector{Int64} at index [3]
Stacktrace:
 [1] getindex(A::Vector{Int64}, i1::Int64)
   @ Base ./essentials.jl:13 [inlined]
 [2] findprev(testf::Base.Fix2{typeof(==), UInt8}, A::Vector{Int64}, start::Int64)
   @ Base ./array.jl:2313
 [3] top-level scope
   @ REPL[28]:1
julia> findprev(==(1), UInt8[1,2], 3)
ERROR: BoundsError: attempt to access 2-element Vector{UInt8} at index [3]
Stacktrace:
 [1] getindex(A::Vector{UInt8}, i1::Int64)
   @ Base ./essentials.jl:13 [inlined]
 [2] findprev(testf::Base.Fix2{typeof(==), Int64}, A::Vector{UInt8}, start::Int64)
   @ Base ./array.jl:2313
 [3] top-level scope
   @ REPL[31]:1
julia> findprev(x->x==(0x1), UInt8[1,2], 3)
ERROR: BoundsError: attempt to access 2-element Vector{UInt8} at index [3]
Stacktrace:
 [1] getindex(A::Vector{UInt8}, i1::Int64)
   @ Base ./essentials.jl:13 [inlined]
 [2] findprev(testf::var"#7#8", A::Vector{UInt8}, start::Int64)
   @ Base ./array.jl:2313
 [3] top-level scope
   @ REPL[33]:1