Add STDF and ATDF file type detection

[nerohmot]

This PR adds support for detecting STDF (Standard Test Data Format) and ATDF (ASCII Test Data Format) files.

Changes:

• Added is_stdf() matcher for both little-endian and big-endian STDF files
• Added is_atdf() matcher for ATDF files (checks for 'FAR:A' signature)
• Added MIME types: application/x-stdf and application/x-atdf
• Added test cases for both formats (sample_le.stdf, sample_be.stdf, sample.atdf)
• Updated README.md with new file types
• Note: STDF matcher placed before COFF matcher to avoid false positive (STDF LE shares 0x00 0x02 prefix with COFF IA64)