refactor(core): extract react.ts from index.ts

[dai-shi]

This splits code into the separate file react.ts.

[codesandbox-ci]

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Latest deployment of this branch, based on commit 2593c91:

Sandbox	Source
React	Configuration
React Typescript	Configuration
React Browserify	Configuration
React Snowpack	Configuration
React Parcel	Configuration
Next.js	Configuration

[github-actions]

Size Change: 0 B

Total Size: 11.3 kB

Filename	Size	Change
dist/esm/index.js	1.22 kB	-3 B (0%)
dist/esm/vanilla.js	521 B	+1 B (0%)
dist/index.js	1.35 kB	-2 B (0%)
dist/vanilla.js	634 B	+4 B (+1%)

ℹ️ View Unchanged

Filename	Size
dist/context.js	622 B
dist/esm/context.js	521 B
dist/esm/middleware.js	2.82 kB
dist/esm/shallow.js	272 B
dist/middleware.js	2.98 kB
dist/shallow.js	318 B

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