Validate extension before packaging

[artrz]

Hello. Similarly to #570, I haven't found a way to validate if an extension is correctly configured before packaging / publishing which makes automation more difficult. I've only been bitten when engines.vscode and @types/vscode dep aren't compatible so I'm not aware of other constraints. Using vsce package leaves leftovers and using a script would require replication it in every extension repository. Is it possible to add something like vsce validate / vsce package --dry-run?