Implement percentage-closer soft shadows (PCSS). #48340

Job	Run time
ci	4m 19s
build (windows-latest)	8m 52s
build (ubuntu-latest)	4m 34s
check-doc	11m 19s
build (macos-latest)	3m 16s
check-missing-examples-in-docs	36s
run-examples-macos-metal	8m 1s
miri	11m 55s
toml	1m 51s
typos	8s
check-bevy-internal-imports	3s
check-missing-features-in-docs	31s
markdownlint	1m 12s
check-compiles	6m 17s
build-wasm	1m 45s
build-wasm-atomics	1m 55s
msrv	1m 45s
	1h 8m 19s

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