webpack: Increase Node.js heap size on low-memory systems.
webpack: Increase Node.js heap size on low-memory systems.

On a 2 GiB, 1 CPU system, webpack would hit the Node.js heap
limit (which is half of physical memory up to 4 GiB, on 64-bit
systems).

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

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