Add application/x-ms-pdb

PDB files contain debug symbols for PE libraries and executables, which can be used with debuggers such as winedbg.

application/x-ms-pdb is not an official IANA media type, but is used by the file utility and others.

https://github.com/file/file/blob/de7d52dce3e7a0bb1a72f299a265c2b641187842/magic/Magdir/msvc https://github.com/microsoft/microsoft-pdb/blob/805655a28bd8198004be2ac27e6e0290121a5e89/PDB/msf/msf.cpp#L961