[BUGFIX BETA CANARY] Add ember-decorators-polyfill dep to @ember-data…

…/store

Extracted from emberjs#6098

emberjs#6249 refactored `Store` to a class.
As part of this refactoring, `Store#defaultAdapter` was implemented
using the `@computed` decorator. Using the `@computed` decorator
requires Ember 3.10+.

This adds [`ember-decorators-polyfill`](https://github.com/pzuraq/ember-decorators-polyfill)
as a dependency to `@ember-data/store` to add support for Ember < 3.10.

packages/store/package.json

@@ -26,6 +26,7 @@
     "ember-cli-babel": "^7.8.0",
     "ember-cli-path-utils": "^1.0.0",
     "ember-cli-typescript": "^2.0.2",
+    "ember-decorators-polyfill": "^1.0.6",
     "heimdalljs": "^0.3.0"
   },
   "devDependencies": {