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Add vcf open mode function to determine VCF/BCF open mode from file extension

Add vcf open mode function to determine VCF/BCF open mode from file extension: a task in LegoFlow-SWE (Harbor dataset). Currently, there is no way to automatically determine the correct file open mode for VCF/BCF files based on the filename, analogous to how sam open mode works for SAM/BAM/CRAM…

The task

Currently, there is no way to automatically determine the correct file open mode for VCF/BCF files based on the filename, analogous to how `sam_open_mode` works for SAM/BAM/CRAM files. When opening variant call format files, the user must explicitly specify the format, which is error‑prone and inconvenient.

Part of Lego-X/LegoFlow-SWE.