fs: support caller-supplied readFile() buffers #7467

Job	Run time
Build slim tarball	1m 7s
aarch64-linux: Cache V8 build	26s
x86_64-linux: with shared libraries	28m 48s
x86_64-darwin: with shared libraries	38m 13s
aarch64-darwin: with shared libraries	26m 40s
aarch64-linux: with shared openssl-3.0.20	23m 51s
aarch64-linux: with shared openssl-1.1.1w	23m 33s
aarch64-linux: with shared openssl-3.6.2	24m 1s
aarch64-linux: with shared openssl-4.0.0	24m 0s
aarch64-linux: with shared boringssl-0.20260508.0	24m 25s
aarch64-linux: with shared openssl-3.5.6	24m 0s
	3h 59m 4s

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