Support generic platform for extensions

[felipecrs]

While the vsce allows publishing to many platforms at one-shot, this would still require the extension authors to keep their list of platforms in sync (i.e. updated). If any new platform is added, their extension would not be available for download in there unless they add this platform to the publish list, even though the extension itself would actually work as the code is platform-agnostic.

The support for a platform named generic would solve this issue, and this is how I believe the VS Marketplace should handle its presence:

1. If an extension for the same platform is published, use it.
2. Otherwise, fallback to generic platform

Refs microsoft/vscode-vsce#625 (comment)
Refs microsoft/vscode#23251 (comment)
Refs microsoft/vscode#23251 (comment)

[felipecrs]

@joaomoreno should this go to the vsce repo instead?

[joaomoreno]

No, support must come from the Marketplace. VSCE is just a tool which operates against it.

[felipecrs]

Right. Thank you!

[SaiKanth007]

Marketplace has the support for publishing platform specific and universal packages against the same version

[felipecrs]

@SaiKanth007 does it mean that I don't need to publish an extension for each specific platform, but only for the ones I support, and VS Code will properly download the Universal one in case the specific platform is missing?

[SaiKanth007]

@felipecrs Yes and I think VSCode Client is already doing it. @sandy081 correct me if my understanding is wrong.