feature: namespace imports and reduce rerenders

[devrnt]

Add namespace imports and reduce IntercomProvider rerenders.

• Reduce bundlesize because of React namespace imports: https://twitter.com/sebmarkbage/status/1250284377138802689
• Using useState in IntercomProvider causes unnecessary rerenders. This is avoided by making use of useRef.

[github-actions]

Size Change: -100 B (1%)

Total Size: 9.25 kB

Filename	Size	Change
dist/react-use-intercom.cjs.development.js	3.46 kB	-70 B (2%)
dist/react-use-intercom.cjs.production.min.js	2.23 kB	-21 B (0%)
dist/react-use-intercom.esm.js	3.42 kB	-9 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
dist/index.js	152 B	0 B

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