Picking a strict packer_version format

[SharkWipf]

It's been raised that the packer_version field should follow a strict format, and not be free-form.
There are plenty of options for this, the most "popular" one nowadays being semver.
From a technical perspective, it would be nice if there is a way to reliably, automatically check if "version x is newer than version y", so you can specify version ranges easily.
Beyond that, I don't hold any strong opinions on this.
Though we probably shouldn't waste a lot of time on this and decide this quickly, this should not become a blocker.

What are your thoughts? Why?

Note: This only applies to packer_version. _created_by_version cannot be constrained, as it pertains to pre-existing applications that already have their own versioning schemes.