feat: publish CLI plugin contract (#107)

We have already assembled this package, but weren't publishing it just
yet. By publishing the CLI contract, plugin implementors will be able to
take a reliable dependency on the contract types, instead of having to
copy/paste source files.

---
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under the terms of the Apache-2.0 license

Author: rix0rrr

.github/workflows/release.yml

@@ -503,9 +503,7 @@ const cdkBuildTools = configureProject(
 // This should be deprecated, but only after the move
 const cliPluginContract = configureProject(
   new yarn.TypeScriptWorkspace({
-    ...genericCdkProps({
-      private: true,
-    }),
+    ...genericCdkProps(),
     parent: repo,
     name: '@aws-cdk/cli-plugin-contract',
     description: 'Contract between the CLI and authentication plugins, for the exchange of AWS credentials',