Make react an optional peer dependency

NPM just launched optional peer dependency support via
`peerDependenciesMeta` (npm/cli#224), so
we can now leverage this to make `react` a trully optional dep,
instead of using NPM's poorly named `optionalDependencies`
(since `optionalDependencies` are really just normal
dependencies). This means people using `@apollo/client` who
aren't interested in using React won't be annoyed by missing
`react` peer dependency warnings from npm.

package.json

@@ -52,11 +52,14 @@
     }
   ],
   "peerDependencies": {
-    "graphql": "^14.5.4"
-  },
-  "optionalDependencies": {
+    "graphql": "^14.5.4",
     "react": "^16.8.0"
   },
+  "peerDependenciesMeta": {
+    "react": {
+      "optional": true
+    }
+  },
   "dependencies": {
     "@types/zen-observable": "^0.8.0",
     "@wry/equality": "^0.1.9",
@@ -84,7 +87,8 @@
     "jest-junit": "8.0.0",
     "lodash": "4.17.15",
     "prop-types": "15.7.2",
-    "react-dom": "^16.10.2",
+    "react": "^16.11.0",
+    "react-dom": "^16.11.0",
     "rimraf": "^3.0.0",
     "rollup": "1.21.2",
     "rollup-plugin-commonjs": "^10.1.0",