Add exports field to react-dom (#23252)

This configures the exports field for react-dom.

Notably there are some conditions for the /server entry point where we pick
the export based on environments. Most environments now support Web Streams
which is preferred in those environments.

We already do this using the "browser" field so the "browser" condition
applies here too.

I don't think it's necessary, but I also specified "worker" explicitly
since this is for Service Workers and those are often targeted with
Web Pack's "webworker" target, which is also what Cloudflare currently
recommends.

I also added "deno" but deno is a bit special because this only works if
you run with the node compatibility since otherwise you have to specify
absolute URLs for the imports.

Author: sebmarkbage

packages/react-dom/package.json

@@ -38,6 +38,19 @@
     "cjs/",
     "umd/"
   ],
+  "exports": {
+    ".": "./index.js",
+    "./server": {
+      "deno": "./server.browser.js",
+      "worker": "./server.browser.js",
+      "browser": "./server.browser.js",
+      "default": "./server.node.js"
+    },
+    "./profiling": "./profiling.js",
+    "./test-utils": "./test-utils.js",
+    "./package.json": "./package.json",
+    "./": "./"
+  },
   "browser": {
     "./server.js": "./server.browser.js"
   },