fs: support caller-supplied readFile() buffers #7893

Job	Run time
Build slim tarball	1m 19s
aarch64-linux: Cache V8 build	21s
x86_64-linux: with shared libraries	27m 15s
x86_64-darwin: with shared libraries	37m 2s
aarch64-darwin: with shared libraries	24m 51s
aarch64-linux: with shared openssl-3.5.6	23m 47s
aarch64-linux: with shared openssl-3.0.20	25m 1s
aarch64-linux: with shared boringssl-0.20260526.0	24m 46s
aarch64-linux: with shared openssl-4.0.0	23m 50s
aarch64-linux: with shared openssl-1.1.1w	23m 35s
aarch64-linux: with shared openssl-3.6.2	24m 13s
	3h 56m 0s

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