src: use `visibility("default")` for exports on POSIX
Electron uses this because Chromium builds with symbols hidden by default.
Refs: https://github.com/electron/node/commit/88b494191c2a5b50b01dab80cd61ba3c0e0fbeb9 Refs: https://github.com/electron/node/commit/1293d1d7d0c33d3925da11ceccdce4eb2e927a43
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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