feat: Add es2015 entries to the exports declaration to support Angular (#6614)

Based on #6613 (comment). Adds `es2015` entries to the `exports` declaration in the `package.json`.

Author: crisbeto

package.json

@@ -17,30 +17,37 @@
   "exports": {
     ".": {
       "node": "./dist/cjs/index.js",
+      "es2015": "./dist/esm/index.js",
       "default": "./dist/esm5/index.js"
     },
     "./ajax": {
       "node": "./dist/cjs/ajax/index.js",
+      "es2015": "./dist/esm/ajax/index.js",
       "default": "./dist/esm5/ajax/index.js"
     },
     "./fetch": {
       "node": "./dist/cjs/fetch/index.js",
+      "es2015": "./dist/esm/fetch/index.js",
       "default": "./dist/esm5/fetch/index.js"
     },
     "./operators": {
       "node": "./dist/cjs/operators/index.js",
+      "es2015": "./dist/esm/operators/index.js",
       "default": "./dist/esm5/operators/index.js"
     },
     "./testing": {
       "node": "./dist/cjs/testing/index.js",
+      "es2015": "./dist/esm/testing/index.js",
       "default": "./dist/esm5/testing/index.js"
     },
     "./webSocket": {
       "node": "./dist/cjs/webSocket/index.js",
+      "es2015": "./dist/esm/webSocket/index.js",
       "default": "./dist/esm5/webSocket/index.js"
     },
     "./internal/*": {
       "node": "./dist/cjs/internal/*.js",
+      "es2015": "./dist/esm/internal/*.js",
       "default": "./dist/esm5/internal/*.js"
     },
     "./package.json": "./package.json"