chore: add bundle files as exportable asset (#682)

This restores the possibility to import the bundle directly, without
getting the following error:
    
> [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './dist/engine.io.min.js' is not defined by "exports"

package.json

@@ -7,6 +7,9 @@
   "module": "./build/esm/index.js",
   "exports": {
     "./package.json": "./package.json",
+    "./dist/engine.io.esm.min.js": "./dist/engine.io.esm.min.js",
+    "./dist/engine.io.js": "./dist/engine.io.js",
+    "./dist/engine.io.min.js": "./dist/engine.io.min.js",
     ".": {
       "import": {
         "node": "./build/esm-debug/index.js",