Add option to adjust thread priorities

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To improve Minecraft client performance on systems with very few hardware cores, it's important to keep the concurrently running thread count low, and also ensure the client thread has highest priority (to avoid stutter). To accomplish this, the following priority changes are made if threadPriorityFix is enabled:

• Client thread given default priority + 2
• Server thread given min priority + 2
• Chunk builder threads given min priority + 1
• Number of chunk builders adjusted to match Sodium defaults on modern versions. On my i3-4150 (with 2c4t) vanilla creates 4 chunk builders by default, which causes huge amounts of stuttering. With these changes it only creates 1, which makes sense, as that brings the total vanilla thread count to 3 not counting whatever mods do in the background.

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Marking this as draft because the chunk builder seems to be running way too slowly.