Support packageJson.preRelease: true along packageJson.version

[hediet]

Conceptually, the pre-release flag is tied to the version number.
However, at the moment, the version is specified in package.json, but the pre-release flag is passed in as cli argument, both for package and publish.
This also makes me wonder what happens when a vsix is packaged with --pre-release, but published without (and vice versa, packaged without and published with).
If the pre-release flag was read from package.json (from source or inside the given vsix file), it would be quite clear.

To fix issues like #947 and make it less confusing, I'd suggest that by default, the pre-release flag is read from a package.json property 'preRelease'.

---

It seems like publish only uses the --pre-release flag if the vsix still needs to be build (i.e. if no vsix is specified).