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.
(cherry picked from commit 8e524c73804a9615dd68011b2c5741947d19bbb6)
