[Turbopack] add feature flag to use the new backend in next.js #34369

Job	Run time
Determine changes	16s
validate-docs-links	18s
build-native / build	4m 11s
build-next / build	30s
test unit (18) / build	4m 2s
test unit (20) / build	4m 14s
ast-grep lint	14s
test next-swc wasi	0s
rust check / build	2m 5s
test cargo benches / Test	4m 24s
rustdoc check / build	1m 52s
test cargo unit / build	2m 31s
test next-swc wasm / build	4m 12s
types and precompiled	0s
Run devlow benchmarks	0s
test turbopack dev	0s
lint	0s
test turbopack development integration	0s
Test new tests for flakes (dev)	0s
test turbopack production	0s
test turbopack production integration	0s
test dev	0s
Test new tests for flakes (prod)	0s
test firefox and safari	0s
test integration	0s
test prod	0s
test ppr dev	0s
test ppr integration	0s
test ppr prod	0s
Test new tests when deployed	0s
report test results to datadog	4s
thank you, next	3s
	28m 56s

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