Previously, `text-ansi` declared a direct `foreign import` on the `isatty`
function. This is subtly incorrect on Windows, which provides an `_isatty`
function rather than `isatty`. (See
https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/posix-isatty?view=msvc-170)

This happened to work on pre-9.4.5 versions of GHC due to them linking against
the `msvcrt` C runtime (which still provided `isatty`), but GHC 9.4.5 and later
link against `ucrt`, which does not provide `isatty` at all. This manifests in
linker errors when using `text-ansi` in GHCi or Template Haskell on Windows, as
seen in https://gitlab.haskell.org/ghc/ghc/-/issues/23378 and in this `hledger`
build:
https://github.com/simonmichael/hledger/actions/runs/4931242306/jobs/8815809083

The solution is to instead declare a foreign import on `_isatty`. The
`c_isatty` function from `System.Posix.Internals` in `base` already does this,
in fact, so I have removed the hand-written `c_isatty` functions in `text-ansi`
in favor of the one in `System.Posix.Internals`. (Despite that module's name,
`System.Posix.Internals` is cross-platform and does in fact work on Windows.)