Create a constellation_traits crate (#36088)

This change creates a `constellation_traits` crate. Previously messages
to the `Constellation` were in the `compositing_traits` crate, which
came about organically. This change moves these to a new crate which
also contains data types that are used in both compositing/libservo and
script (ie types that cross the process boundary). The idea is similar
to `embedding_traits`, but this is meant for types not exposed to the
API.

This change allows deduplicating `UntrustedNodeAddress`, which
previously had two versions to avoid circular dependencies.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/shared/script_layout/Cargo.toml

@@ -16,6 +16,7 @@ base = { workspace = true }
 app_units = { workspace = true }
 atomic_refcell = { workspace = true }
 canvas_traits = { workspace = true }
+constellation_traits = { workspace = true }
 euclid = { workspace = true }
 fnv = { workspace = true }
 fonts = { path = "../../fonts" }