Packaging should lint manifest files with vscode's extension linter and show warnings & errors

[ulugbekna]

Currently, we have an extension linter, which shows warnings & errors when an extension manifest file (package.json) is open. But an extension author should open the manifest file to see those diagnostics. I think it's worth showing the warnings & errors when running vsce package

This would notify extension authors about their manifest file problems & encourage to fix them.

(this's problematic to implement because it would require the linter to be a standalone npm package, but the linter has dependenices on vscode core)

[joaomoreno]

Love this idea.