`Alignment` type needs some improvement along with definition of "anchors"

[keighrim]

Because

This came up in a in-person meeting the other day (and still quite abstract and general goal); we might want a general re-design of anchors and alignment type for better clarity and guidance on when to use what.

A few keywords from the in-person discussion;

• what to do with multi-modal "anchor" types (Polygon subtypes with timePoint and coords)
• rename targets prop to elements?
• Can anchors of the same modality be aligned? (TextDocument <-> TextDocument for derived texts)

Done when

I don't know that this has a clear "goal" yet, and I think that there needs to be continued attention at various levels to narrow down the scopes of the different concepts within the CLAMS vocabulary.

Additional context

No response