Change `#Lineage.joinSchema` from `_` (top) to open struct with version

[sdboyer]

#Lineage.joinSchema defines the join/least upper bound that must be maintained by all schemas in a lineage. Currently, it starts as top, _:

https://github.com/grafana/thema/blob/b7b433093f2fd56c11e68a45b0c3ee7b7850ca8c/lineage.cue#L11-L24

This isn't great:

• It makes injecting a themaVersion field complicated, since we can't universally assume all schemas are a struct at base (could be scalar or list)
  • We can't even assume that all schemas are structs within a lineage
• It's plausible that other bits of thema's helper CUE logic may be subtly assuming a struct type here as well
• I don't see a use case for scalars or lists as a base schema type, for essentially the same reasons that

I'm sure i want it to be an open struct, {...}. i was worried about open/closedness here, but after some poking on the playground, i think we can make the shift without forcing any decisions on openness, which is great - that's a can of worms.

I don't see an immediate reason why this change would be anything other than changing just the one line - i'm mostly writing this issue up for posterity and as a place to dump that link to the playground. (also have it in a gist)

[sdboyer]

Thinking on this further, it may be that we want to actually inject this instead:

joinSchema: {
    themaVersion?: [int & >= 0, int & >= 0]
    ...
}

But i'll probably leave that for a follow-up

[sdboyer]

When talking all this through with @Lyoness and @myitcv earlier today, i realized that this, the possibility of adding the themaVersion? field, and #76 can be taken care of all in one motion by something like this:

1. Making joinSchema into {...} as this originally proposed, and renaming it to `allSchem
2. Adding an allSchemasMust, which is a struct that must be valid when unified against all schemas in the lineage, but the result of that unification is not part of the actual used schema.