improved build speed of main.js

[keVIMena98]

Running the build command to generate 10,000 images can take a pretty long while to finish. This is true especially if you are running this program o a low budget computer with very little memory.

I noticed that the saveImage function is utilizing a writeFileSync in node is an asynchronous function that blocks the entire thread until the file is done writing to disc. To resolve this just converted this to a promise and moved some code around.

Another but very minimal change was adjusting for the deprecation of fs.rmdirSync(buildDir, { recursive: true }); by instead using fs.rmSync(buildDir, { recursive: true });.

[DaWe35]

TLDR: it is actually slower while eating RAM like insane

I tested it and generated 20 images.

Env:
Windows 11
i5-10200H
Micron 2210 NVMe SSD
16 GB RAM
9+ image layers, 4 variations

Results:
Sim-Labs-LLC:build_speed - 2:44
HashLips:main - 2:34

With Sim-Labs-LLC:build_speed, for some reason, image weren't showing up in the image folder until the code finished.
My RAM usage also went up to an insane 99% level (terminal used 10 GBs of RAM alone), compared to 3 GB on hashlips:main)

So overall, this PR not only made my build time slower, it has a memory leak too. I can image a situation when Sim-Labs-LLC:build_speed is faster, but I couldn't get why would it be better for low-RAM computers.

[nhonx]

I think instead of blindly converting asynchronous writeFileSync to Promise, which can cause RAM eating, we should rewrite the code to allow writing file parallelly in multiple threads.
TLDR:
Original: write file one by one
OP solution: write all file at once
Suggesting: write X files a times, X can be configured depends on the machine.