Add a similar method for Type{<:CodeUnits} (#57826)

Currently, `similar(::CodeUnits)` works as expected by going through the
generic `AbstractArray` method. However, the fallback method hit by
`similar(::Type{<:CodeUnits}, dims)` does not work, as it assumes the
existence of a constructor that accepts an `UndefInitializer`. This can
be made to work by defining a corresponding `similar` method that
returns an `Array`.

One could make a case that this is a bugfix since it was arguably a bug
that this method didn't work given that `CodeUnits` is an
`AbstractArray` subtype and the other `similar` methods work. If anybody
buys that argument, it could be nice to backport this; it came up in
some internal code that uses Arrow.jl and JSON3.jl together.

Author: ararslan

base/strings/basic.jl

@@ -805,6 +805,8 @@ write(io::IO, s::CodeUnits) = write(io, s.s)
 cconvert(::Type{Ptr{T}},    s::CodeUnits{T}) where {T} = cconvert(Ptr{T}, s.s)
 cconvert(::Type{Ptr{Int8}}, s::CodeUnits{UInt8}) = cconvert(Ptr{Int8}, s.s)
 
+similar(::Type{<:CodeUnits{T}}, dims::Dims) where {T} = similar(Array{T}, dims)
+
 """
     codeunits(s::AbstractString)
 

test/strings/basic.jl

@@ -1096,6 +1096,7 @@ let s = "∀x∃y", u = codeunits(s)
     @test_throws Base.CanonicalIndexError (u[1] = 0x00)
     @test collect(u) == b"∀x∃y"
     @test Base.elsize(u) == Base.elsize(typeof(u)) == 1
+    @test similar(typeof(u), 3) isa Vector{UInt8}
 end
 
 @testset "issue #24388" begin