Enable corepack if packageManager is defined

[fcollonval]

Fixes #240

packageManager has been added to package.json to test this.

[fcollonval]

@krassowski @jtpio with the rolling of braces update by dependabot. I see lots of CI failing because the yarn.lock format gets updated. This is a side effect of the yarn version used by dependabot.

This did not happen on Lumino upgrade PR. I think because the packageManager is set in package.json. Therefore I would like to use that trick to prevent dependabot of running an unsupported version of yarn. Hence the need for this PR.

[jtpio]

Thanks @fcollonval for looking into this!

Indeed, these failures have also been happening on other extension repos in general. Enabling corepack sounds like a good way to mitigate this (not sure there is another way).

[fcollonval]

@jtpio FYI it fixed the issue with dependabot: https://github.com/jupyterlab-contrib/jupyterlab-variableInspector/pull/303/files