Dependencies: Drop-in replacement for std::filesystem (in extern/)

Introduces the ghc::filesystem library as a (leight weight, header
only) drop-in replacement for std::filesystem. The latter can't be
used on the minimum macOS version we support. Once that changes, we can
switch to using std::filesystem directly.

Part of T90379.

* Add ghc::filesystem to extern/ as platform compatible replacement for
std::filesystem.
* Add Windows specific code to avoid issues with including Windows.h.
* Add blender::bli::filesystem as alias for ghc::filesystem. That should make
the ghc -> std transition easier later on since we can just change this
alias. Also makes it easy to switch between the two at compile time.