feat: exports the module of extension in package.json

Author: shijistar

package.json

@@ -78,6 +78,10 @@
       "types@>=5": "./lib/types.d.ts",
       "types": "./types-legacy/pre-v5/types.d.ts"
     },
+    "./extension": {
+      "types@>=5": "./lib/extension.d.ts",
+      "types": "./types-legacy/pre-v5/extension.d.ts"
+    },
     "./localize-interface": {
       "types@>=5": "./lib/localize-interface.d.ts",
       "types": "./types-legacy/pre-v5/localize-interface.d.ts"