refactor: move klib out of src/nvim/ #20341

It's confusing to mix vendored dependencies with neovim source code. A
clean separation is simpler to keep track of and simpler to document.

src/nvim/context.h

@@ -3,8 +3,8 @@
 
 #include <msgpack.h>
 
+#include "klib/kvec.h"
 #include "nvim/api/private/defs.h"
-#include "nvim/lib/kvec.h"
 
 typedef struct {
   msgpack_sbuffer regs;     ///< Registers.