Require sharp from the main thread to prevent issues on worker exit

[devongovett]

Potentially fixes #5961

Sharp doesn't play nice with Node worker_threads, because on thread exit it may unload a shared library that is still being used. So this tries to require it from the main thread before using it in a worker. See https://sharp.pixelplumbing.com/install#worker-threads and lovell/sharp#2263

[height]

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