DevTools: Fix Fiber leak caused by Refresh hot reloading

[bvaughn]

Resolves #21469 as well as #21436 and #21442 (I think)

Posting for discussion purposes with @gaearon

The general idea is to add a new hook that Fast Refresh can use to inform DevTools when a force remount is scheduled. Then DevTools can clean up any Fibers that would have otherwise been linked by the clone-and-splice operation on the tree.

Added some basic integration tests for Fast Refresh + DevTools, including one that fails without this fix:

[sizebot]

Comparing: 46491dc...de95ad8

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	126.12 kB	126.12 kB	=	40.46 kB	40.46 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	128.94 kB	128.94 kB	=	41.40 kB	41.40 kB
facebook-www/ReactDOM-prod.classic.js	=	406.40 kB	406.40 kB	=	75.19 kB	75.19 kB
facebook-www/ReactDOM-prod.modern.js	=	394.46 kB	394.46 kB	=	73.29 kB	73.29 kB
facebook-www/ReactDOMForked-prod.classic.js	=	406.40 kB	406.40 kB	=	75.19 kB	75.19 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against de95ad8