Export isDraft from Immer

[markerikson]

This PR:

• Removes the perf benchmark file I forgot to nuke from Optimize dev check middleware performance #943
• Re-exports isDraft from Immer, since we already did original
• Updates the docs accordingly

[github-actions]

Size Change: +32 B (0%)

Total Size: 69.8 kB

Filename	Size	Change
dist/redux-toolkit.cjs.development.js	13.9 kB	+10 B (0%)
dist/redux-toolkit.cjs.production.min.js	5.3 kB	+10 B (0%)
dist/redux-toolkit.esm.js	13.8 kB	+1 B
dist/redux-toolkit.umd.js	25.7 kB	+6 B (0%)
dist/redux-toolkit.umd.min.js	11 kB	+5 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
dist/index.js	149 B	0 B

_{compressed-size-action}

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