Convert to rattler-build recipe; make ghostscript an extra (optional dependency group)

[jonashaag]

Use v3 repo data to ship Ghostscript as optional dependency

This converts the recipe to the rattler-build v1 format and uses the new repodata v3 optional dependency groups (CEP under vote, open until 2026-08-20) to make ghostscript an explicit opt-in for the permissive variants:

• agpl variant: unchanged — keeps the unconditional ghostscript run dependency (it links libgs, which is what makes it AGPL).
• imagemagick variant: no longer hard-depends on ghostscript at runtime. It never links libgs (the existing test asserts gslib is absent from delegates); gs was only used as an external delegate subprocess. Users who want gs-backed PDF/PS/EPS conversion opt in via imagemagick[extras=[ghostscript]] (or by installing ghostscript themselves).

Default solves are unaffected: the +100 build-number offset still makes solvers prefer the agpl variant, which behaves exactly as before. Only users explicitly pinning build=imagemagick_* are affected, and for them shipping the AGPL gs binary implicitly arguably contradicts the point of that variant.

Draft because: requirements.extras requires rattler-build's --v3 flag, which conda-forge does not enable yet. Rendering was validated locally with rattler-build build --render-only --v3 against conda_build_config.yaml; both variants render with the expected build strings (agpl_h*_101, imagemagick_h*_1), run dependencies, and the extras group. Once conda-forge rolls out v3 output this can be un-drafted and rerendered.

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I wanted to let you know that I linted all conda-recipes in your PR (recipe/recipe.yaml) and found some lint.

Here's what I've got...

For recipe/recipe.yaml:

• ❌ The recipe license should not include the word "License".

For recipe/recipe.yaml:

• ℹ️ License is not an SPDX identifier (or a custom LicenseRef) nor an SPDX license expression.

Documentation on acceptable licenses can be found conda-forge.org > Docs > Maintainer Documentation > Contributing packages > SPDX Identifiers and Expressions.

• ℹ️ Found bld.bat in recipe directory, but this is a recipe v1 (rattler-build recipe). rattler-build uses build.bat instead of bld.bat for Windows builds. Consider renaming bld.bat to build.bat.

_{This message was generated by GitHub Actions workflow run https://github.com/conda-forge/conda-forge-webservices/actions/runs/31179030685. Examine the logs at this URL for more detail.}