cmake,ci: vcpkg for MSVC default, fix lz4 include, catch2, cleanup

vcpkg: Add Catch2 2.13.9

Catch2 >= 3.0 is not compatible with earlier versions, and for now we
must override the desired version in our vcpkg manifest. We can do this
programmatically by using VCPKG_MANIFEST_FEATURES.

CMakeLists: Search for lz4 CONFIG mode first

vcpkg's lz4 CONFIG cmake script works in Release mode but not in Debug
mode, failing to copy the correct DLLs at compile time.

We still need to search for the regular mode for system-installed
versions.

CMakeLists: Clean up boost exports

Remove some Conan-specific workarounds.

CMakeLists: Use vcpkg for MSVC by default

Not enabling it generally since it's much easier to have system
dependencies installed for Linux and MinGW.