Add development headers (iapi.h, ierrors.h) to support downstream packages

### Problem

The current `ghostscript` package does not include development headers (`ghostscript/iapi.h`, `ghostscript/ierrors.h`), which are required for downstream packages to link against `libgs`.

As a result, ImageMagick's configure step falls back silently:

```log
Ghostscript lib --with-gslib=yes no (failed tests)
```

Even though `libgs.so` is present, the missing headers cause the completeness check to fail:

```log
checking for ghostscript/iapi.h... no
checking for ghostscript/ierrors.h... no
checking if Ghostscript package is complete... no -- some components failed test
```

ImageMagick then falls back to invoking `gs` as an external command instead of linking `libgs` directly.

### Proposed Fix

Split the package into two outputs following conda-forge conventions:

- `ghostscript` — runtime package (binaries, `libgs.so`, fonts)
- `ghostscript-devel` — development package (headers, pkg-config files)

This allows downstream packages to express:

```yaml
host:
- ghostscript-devel
run:
- ghostscript
```

### Affected Downstream Packages

- `imagemagick` (conda-forge/imagemagick-feedstock) — confirmed affected