This method is an ActiveSupport extension, but there's no harm in
recognising it universally as any identically-named method is likely to
also be a file writer.
The main goal here is to get rid of the duplicate definitions of module
`IO`, which currently exist in both `frameworks/core/IO.qll` and
`frameworks/Files.qll`.
We do this by moving the classes inside `Files::IO` to `core/IO.qll`,
but moving most of the actual definitions of those classes to an
internal module `core.internal.FileOrIO`. This means both `Files.qll`
and `IO.qll` can depend on them without leaking them to end users.
