[Feature request] Support publishing both universal and target-specific versions

Nobody has claimed this yet.

`ms-python.debugpy` provides both universal and target-specific versions, but the universal version is not presented on https://open-vsx.org.

In this case, as https://open-vsx.org is used as the default extension gallery for VSCodium, on platforms where VSCodium supports but open-vsx doesn't (e.g. LoongArch), users aren't able to search and download said extension directly.

While I was trying to add such support here I found that the [schema file in this repository](https://github.com/EclipseFdn/publish-extensions/blob/bc9c9437af959e9ae1ebc784e053aec9bf54a2b3/extensions-schema.json) doesn't allow publishing both universal and target-specific extensions sharing the same ID.

I would suggest adding such support to the schema and scripts, so that it would be possible to publish and present both the universal and target-specific versions of extensions like `ms-python.debugpy`.

Start with extensions-schema.json, then trace the publishing scripts that consume it. Confirm how extension IDs and target-specific versions are currently validated and presented. Done means the schema and scripts accept both universal and target-specific versions sharing an ID, while preserving current publishing behavior.

Written by the indexing model from the issue text.