feat: declare namespace printing support for Flow (#16066)

tests/unit/__snapshots__/visitor-keys.js.snap

@@ -200,6 +200,10 @@ exports[`visitor keys estree 1`] = `
   "DeclareModuleExports": [
     "typeAnnotation",
   ],
+  "DeclareNamespace": [
+    "id",
+    "body",
+  ],
   "DeclareOpaqueType": [
     "id",
     "typeParameters",