FEXCore: Removes C wrapper interface

This has been a long time coming. The C interface has been a thorn in
our side for no reason for a long time.

The purpose of this step is to remove the C interface without changing
behaviour as much as possible. This means that with this commit there
are still some bad practices but the remaining issues will be solved
with followup PRs.

Primarily, we still have a `DestroyContext(CTX)` static function which calls
the Context implementation's `DestroyContext` and does a raw C++ delete.

Follow up PR will remove that, but I didn't want to touch it yet since
it'll require checking to ensure the unique_ptr changes play nice with
our allocator hooking. Which this is already a huge PR without trying to
change behaviour.

External/FEXCore/Source/Interface/Config/Config.cpp

@@ -30,7 +30,7 @@
 #include <tiny-json.h>
 
 namespace FEXCore::Context {
-  struct Context;
+  class Context;
 }
 
 namespace FEXCore::Config {