[css-contain-3] Define a syntax for state-based container queries

In the discussion of what container features can be queried, there was interest in queries based on the "current state" of a container – such as whether a position: sticky container is currently "stuck" (displaced from its in-flow position), or whether the box is currently visible on screen.

In the draft of CSS Containment L3 we currently allow a state value for container-type, but different states may require different types of containment (or none at all), so I don't think that single type value will work. We also need to define which states are supported initially, and how these states are queried in the @container rule.

For example, something along the lines of:

header {
  container: is-stuck is-visible / header;
  position: sticky;
  top: 0;
}

@container header (is-stuck) { /* … */ }
@container header (is-visible) { /* … */ }

Start by reading the discussion of queryable container features in issue #5989 and the CSS Containment Level 3 draft's container-type section. Review the proposed sticky and visible examples, then establish an agreed syntax, supported initial states, and how those states are queried in @container.