Use `libnpmversion` instead of execing `npm version`

[felipecrs]

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