Add deprecation warning to Ember.assign

[nlfurniss]

Ember RFC #750 was approved; this implements it.

The url for the deprecation is optimistic (in that that page does not yet exist), let me know if I should handle that differently.

[nlfurniss]

Rebased and pushed. Not sure why lint failed on a file I didn't edit 🤨

[rwjblue]

Only one small tweak here, but I'd love another PR that actually removes the implementation in packages/@ember/polyfills/lib/assign.ts. Now that we only support browsers that include Object.assign we can basically do:

export default Object.assign;

(also removing the babel-plugin-transform-object-assign usage)

[nlfurniss]

Only one small tweak here, but I'd love another PR that actually removes the implementation in packages/@ember/polyfills/lib/assign.ts. Now that we only support browsers that include Object.assign we can basically do:

export default Object.assign;

(also removing the babel-plugin-transform-object-assign usage)

Added that here, since it seems like it should be in one PR, but happy to move if you'd like.

and the babel transform was removed in #19563

[mixonic]

Looks good to me. I rebased to resolve a conflict and force pushed.