Update Immer to 8.x

[markerikson]

Fixes #809 .

This PR:

• Bumps shelljs to 0.8.4 to fix some annoying import warnings
• Adds a test that specifically imports Immer with NODE_ENV === 'production', then tries to mutate state from a reducer and expects an error. Under Immer 7, that test fails because it's an unfrozen POJO and the mutation runs without error
• Updates Immer to 8.x. Since Immer 8 autofreezes in prod, the test throws an error as expected and now passes.

My one concern atm is the Immer docs advice to consider "pre-freezing large data ahead of time":

https://immerjs.github.io/immer/docs/freezing

do we need to note this ourselves somewhere? should we be re-exporting freeze() or something?

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Size Change: +158 B (0%)

Total Size: 64.4 kB

Filename	Size	Change
dist/redux-toolkit.umd.js	24.3 kB	+78 B (0%)
dist/redux-toolkit.umd.min.js	10.4 kB	+80 B (0%)

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Filename	Size	Change
dist/index.js	149 B	0 B
dist/redux-toolkit.cjs.development.js	12.4 kB	0 B
dist/redux-toolkit.cjs.production.min.js	4.74 kB	0 B
dist/redux-toolkit.esm.js	12.4 kB	0 B

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[markerikson]

Uh... CI blew up on this push with some unrelated TS errors. Something about the integration branch, maybe?