Llama-quantize: Partial requant feature (#1313)

* Partial Requant feature for llama-quantize

- Inspired by the recently portcopied --dry-run feature.
- Allows to partially requantize a split quantized .gguf by requantizing only the missing splits in the destination directory.
- Works both for GGUF which are split tensors by tensors, or by group of several tensors (though this one is not very much tested beyond 2 tensors by split).
- Vibe coded.

* Create output directory if it doesn't exist in llama-quantize

* Create output directory if it doesn't exist in gguf-split

* Add exit when directory fails to be created on Windows

* Use std::filesystem

* cleanup

include/llama.h

@@ -491,6 +491,7 @@ extern "C" {
         bool ignore_imatrix_rules;           // If set to true, the built-in rules for refusing to quantize into certain quants without imatrix are ignored
         bool only_repack;                    // Only repack tensors
         bool dry_run;                        //
+        bool partial_requant;                // quantize only missing split files in the split quantized .gguf destination directory
         void * imatrix;                      // pointer to importance matrix data
         void * kv_overrides;                 // pointer to vector containing overrides
         void * custom_quants;                // pointer to vector containing custom quantization rules