Add pnpm as a package manager

[nhedger]

Description

This PR adds support for pnpm as a package manager.

Closes #324

Changes

Based on the existing tests, I've created additional tests to validate support for pnpm with all relevant templates.

While implementing the sample notebook renderer with pnpm test, I had to update the ext-notebook-renderer template because the package manager was hardcoded to npm. It will now be replaced by the selected package manager, as in other templates.

Modifying the ext-notebook-renderer template initially broke the sample notebook renderer test because the prompt defined yarn while it expected to find npm. I fixed that by replacing the prompt with npm.

Checklist

• Tests cover pnpm usage
• CLI help has been updated to include pnpm
• CLI prompt has been updated to include pnpm
• Readme has been updated to include pnpm

[ghost]

All CLA requirements met.

[nhedger]

Hey @aeschli,

Friendly ping about this PR.

Is anything else needed from me to move forward with it?

[aeschli]

Thanks @nhedger !

[awxiaoxian2020]

I think we should revert this. Because vsce doesn't support pnpm. See microsoft/vscode-vsce#421 (comment)