feature: make a CUE module

In preparation for publishing to the CUE Central Registry, but also
useful for anyone else who wants to depend on this schema.

Signed-off-by: Paul Jolly <paul@myitcv.io>

Author: myitcv

cue.mod/module.cue

@@ -0,0 +1,7 @@
+module: "github.com/ossf/security-insights-spec"
+language: {
+	version: "v0.11.0"
+}
+source: {
+	kind: "git"
+}