Migrate to using '@types/vscode' rather than 'vscode'

[Kummallinen]

The vscode package was split into '@types/vscode' & '@vscode/test-electron', see https://code.visualstudio.com/updates/v1_36#_splitting-vscode-package-into-typesvscode-and-vscodetest

Using '@types/vscode' provides better build compatibility when behind strict corporate firewalls as it does not need to download vscode.d.ts as a separate step.

[jonahgraham]

Thanks @Kummallinen - this looks like a good idea. It is one of the updates that isn't caught with standard tools.