fs: support caller-supplied readFile() buffers #7814

Job	Run time
Build slim tarball	1m 55s
aarch64-darwin: with shared libraries	27m 50s
x86_64-darwin: with shared libraries	37m 30s
aarch64-linux: Cache V8 build	23s
x86_64-linux: with shared libraries	25m 27s
aarch64-linux: with shared openssl-3.5.6	22m 57s
aarch64-linux: with shared openssl-3.6.2	22m 54s
aarch64-linux: with shared openssl-1.1.1w	21m 59s
aarch64-linux: with shared boringssl-0.20260526.0	22m 57s
aarch64-linux: with shared openssl-3.0.20	23m 5s
aarch64-linux: with shared openssl-4.0.0	22m 57s
	3h 49m 54s

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