MSYS2: Add support for building a stand-alone NSIS installer

MSYS2: Add support for building a stand-alone NSIS installer

This changes the existing code for the MSVC installer as little
as possible to allow building the Wireshark .exe Windows installer
using the MinGW-w64 toolchain.

Currently the DLL dependency list is static, this may change in
the future. Ideally we would use CPack and install() logic
to copy the DLLs.

The msys2checkdeps.py script is copied from the Inkscape project[1].
It doesn't have a specific license identifier. The Inkscape project
is licensed under the GPL version 2 or later.

TODO: Download Npcap and USBPcap using CMake instead of requiring
manual action.

[1]https://gitlab.com/inkscape/inkscape

Ping #17771.