opt: decouple header extraction from file access

[dkogut1996]

No description provided.

[biw]

Looks good to me. Makes things a lot faster!

Before (node 20):

PASS test/index.test.ts (18.78 s)
  gguf
    ✓ Llama-2-7b-Chat-GGUF  (4106 ms)
    ✓ Nous-Capybara-7B-v1.9-GGUF (3834 ms)
    ✓ Xwin-MLewd-7B-V0.2-GGUF (3929 ms)
    ✓ EveryoneLLM-7b.gguf (3734 ms)

After (node 20):

PASS test/index.test.ts (5.639 s)
  gguf
    ✓ Llama-2-7b-Chat-GGUF (888 ms)
    ✓ Nous-Capybara-7B-v1.9-GGUF (408 ms)
    ✓ Xwin-MLewd-7B-V0.2-GGUF (192 ms)
    ✓ EveryoneLLM-7b.gguf (265 ms)
    ✓ L3-8B-Stheno-v3.3-32K.Q8_0.gguf (736 ms)