Add support for CSS backgroundImage gradients on native

[necolas]

Delegate backgroundImage to experimental_backgroundImage in React Native. This should provide support for using linear and radial gradients.

[github-actions]

workflow: benchmarks/size

Comparison of minified (terser) and compressed (brotli) size results, measured in bytes. Smaller is better.

Results	Base	Patch	Ratio
react-strict-dom/dist/dom/index.js
· compressed	2,514	2,514	1.00
· minified	8,691	8,691	1.00
react-strict-dom/dist/dom/runtime.js
· compressed	855	855	1.00
· minified	2,435	2,435	1.00
react-strict-dom/dist/native/index.js
· compressed	16,065	16,021	1.00	-
· minified	59,912	59,998	1.00	+

[github-actions]

workflow: benchmarks/perf (native)

Comparison of performance test results, measured in operations per second. Larger is better.

Results	Base	Patch	Ratio
css.create
· small	1,126,144	1,137,278	1.01	+
· small with units	434,472	429,162	0.99	-
· small with variables	659,796	663,546	1.01	+
· several small	317,955	315,550	0.99	-
· large	205,732	203,112	0.99	-
· large with polyfills	144,063	144,145	1.00	+
· complex	99,570	98,698	0.99	-
· unsupported	217,974	214,954	0.99	-
css.createTheme
· simple theme	223,173	225,012	1.01	+
· polyfill theme	209,622	210,240	1.00	+
css.props
· small	278,902	282,379	1.01	+
· small with units	222,968	223,488	1.00	+
· small with variables	113,879	114,402	1.00	+
· small with variables of units	79,692	79,416	1.00	-
· large	119,212	121,287	1.02	+
· large with polyfills	41,775	41,558	0.99	-
· complex	25,575	25,436	0.99	-
· unsupported	178,049	177,724	1.00	-
· simple merge	185,257	185,781	1.00	+
· wide merge	19,441	19,421	1.00	-
· deep merge	18,941	19,058	1.01	+
· themed merge	37,243	37,005	0.99	-

[NickGerleman]

FYI @intergalacticspacehighway