Use `libnpmversion` instead of execing `npm version`

Hi, I just noticed that npm now publishes `libnpmversion` as a separate library.

I believe vsce could use it so that it no longer needs to call `npm` or `yarn` under the hoods (potentially dropping the need of --yarn as well?)


https://npm.im/libnpmversion
https://github.com/npm/cli/tree/latest/workspaces/libnpmversion
