Add getRootNode to fabric fragment instance

[jackpope]

Stacked on #34533 for root fragment handling

This is the same approach as DOM, where we call getRootNode on the parent.

Tests are in react-native using Fantom.

[react-sizebot]

Comparing: 8bb7241...1ca267f

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.js	=	6.68 kB	6.68 kB	=	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	534.43 kB	534.43 kB	=	94.33 kB	94.33 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	663.69 kB	663.69 kB	=	117.00 kB	117.00 kB
facebook-www/ReactDOM-prod.classic.js	=	687.59 kB	687.59 kB	=	121.04 kB	121.04 kB
facebook-www/ReactDOM-prod.modern.js	=	678.02 kB	678.02 kB	=	119.40 kB	119.40 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 1ca267f