Avoid bundling in ponyfill for Object.assign in use-subscription package

[Andarist]

I've noticed that distributed use-subscription contained code object-assign's code. As you keep it external in case of other packages it makes sense to do the same here.

Just adding it as a dependency in package.json fixes the problem because your Rollup scripts read that and put those into externals option.

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[sizebot]

Size changes (stable)

Details of bundled changes.

Comparing: f4148b2...2b4cfe6

use-subscription

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
use-subscription.production.min.js	-49.6%	-46.0%	1.71 KB	884 B	921 B	497 B	NODE_PROD
use-subscription.development.js	-28.8%	-28.4%	7 KB	4.99 KB	2.73 KB	1.95 KB	NODE_DEV

Generated by 🚫 dangerJS against 2b4cfe6

[sizebot]

Size changes (experimental)

Details of bundled changes.

Comparing: f4148b2...2b4cfe6

use-subscription

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
use-subscription.development.js	-28.7%	-28.2%	7.01 KB	5 KB	2.73 KB	1.96 KB	NODE_DEV
use-subscription.production.min.js	-49.2%	-45.6%	1.73 KB	897 B	930 B	506 B	NODE_PROD

Generated by 🚫 dangerJS against 2b4cfe6

[bvaughn]

Yeah I guess this makes sense, since React is already pulling in this dep!