Support for pnpm

[idleberg]

ovsx publish does not explicitly support the pnpm package manager. To my surprise, using the --yarn flag will publish the extension just fine.

Questions:

1. is using the --yarn flag for packages with pnpm lockfiles intentional (or coincidence?)
2. should the flag be renamed to something more generic?
3. should there be a second flag for pnpm in a future release?
4. will the above still be relevant, once corepack is stable and ships with npm?

[spoenemann]

We just pass that flag to vsce, which is used behind the scenes to package extensions. If vsce adds a dedicated flag for pnpm or changes the --yarn flag, we should adapt ovsx as well.

[idleberg]

Related but unresolved vsce issue:

microsoft/vscode-vsce#421