Support `vcpkg`

[johnpatek]

I want this repo to support building using vcpkg the cmake system can detect if it's vcpkg using the following code:

# Check if CMAKE_TOOLCHAIN_FILE is set
if (DEFINED CMAKE_TOOLCHAIN_FILE)
    message(STATUS "CMAKE_TOOLCHAIN_FILE is set to: ${CMAKE_TOOLCHAIN_FILE}")

    # Check if the path points to the vcpkg toolchain file
    if ("${CMAKE_TOOLCHAIN_FILE}" MATCHES ".*[/\\\\]scripts[/\\\\]buildsystems[/\\\\]vcpkg.cmake$")
        message(STATUS "vcpkg toolchain file detected via path matching.")
        set(VCPKG_TOOLCHAIN_ACTIVE TRUE)
    endif()
endif()

And if it is not, proceed with old FetchContent approach. Otherwise, we should:

1. Build megamime as an overlay port
2. Find mio, argparse, sqlite, etc. using vcpkg.