[routing-utils] add isInternal flag (#9761)

Adds a new Route property called `isInternal` that indicates whether or not this is a project route or a Vercel platform route. Routes with this flag will be hidden from project-specific lists of routes to avoid confusion.

Verified on staging.

packages/routing-utils/src/schemas.ts

@@ -113,6 +113,9 @@ export const routesSchema = {
           check: {
             type: 'boolean',
           },
+          isInternal: {
+            type: 'boolean',
+          },
           status: {
             type: 'integer',
             minimum: 100,