[clangd][vscode] Add npm helper commands to package/release the extension.

Summary: Help to fix https://github.com/clangd/clangd/issues/159.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68080

llvm-svn: 372980