Fix npm post install to fetch extension dependencies

[nhooyr]

Closes #1961

[code-asher]

It seems that the final package.json that VS Code ships for each builtin extension has the dependencies portion removed so yarn ends up not installing anything.

We could prevent that deletion: https://github.com/microsoft/vscode/blob/1.48.2/build/lib/extensions.ts#L73

Or we could only add back the dependencies specifically marked as external in the extension's webpack configuration since the rest will have been bundled.

[nhooyr]

We could prevent that deletion: https://github.com/microsoft/vscode/blob/1.48.2/build/lib/extensions.ts#L73

That's what I'll do!