fix(pkg): add types field for exports (#89)

Author: meteorlxy

package.json

@@ -4,8 +4,13 @@
   "description": "A unified javascript build system",
   "repository": "unjs/unbuild",
   "license": "MIT",
-  "exports": "./dist/index.mjs",
-  "types": "dist/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./dist/index.d.ts",
+      "import": "./dist/index.mjs"
+    }
+  },
+  "types": "./dist/index.d.ts",
   "bin": {
     "unbuild": "./dist/cli.mjs"
   },