Split out core lato crate

[nicoburns]

This PR splits velato into two crates:

• lato which contains everything except the vello integration
• velato which wraps (and re-exports most of) lato and adds vello integration.

This allows non-vello consumers of velato (e.g. AnyReneder / imaging) to depend only on lato and avoid having vello in the dependency tree entirely (even as an optional dependency).

[waywardmonkeys]

Can just delete the vello usage instead, not rename / move things.

[waywardmonkeys]

It is easier. Can do a separate example that shows the very few lines of code needed. Also means it doesn't have to be tied to Vello updates or anything.