Add getComputedStyle helper to ViewTransition refs

[sebmarkbage]

This is also sometimes useful to read the style of the pseudo-element itself without an animation.

[react-sizebot]

Comparing: 4845e16...dadfaac

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	1.82 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	515.14 kB	515.14 kB	=	91.74 kB	91.73 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	+0.02%	614.35 kB	614.48 kB	+0.02%	108.76 kB	108.78 kB
facebook-www/ReactDOM-prod.classic.js	+0.02%	650.34 kB	650.47 kB	+0.01%	114.79 kB	114.81 kB
facebook-www/ReactDOM-prod.modern.js	+0.02%	640.62 kB	640.75 kB	+0.02%	113.23 kB	113.24 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

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