restructure into smaller files & add IWYU/clang-tidy

Signed-off-by: Folling <mail@folling.io>

include/ikarus/scopes/entity_scope.h

@@ -0,0 +1,26 @@
+#pragma once
+
+/// \file entity_scope.h
+/// \author Folling <mail@folling.io>
+
+#include <ikarus/id.h>
+#include <ikarus/macros.h>
+
+/// \addtogroup object_scopes ObjectScopes
+/// @{
+
+IKARUS_BEGIN_HEADER
+
+/// \brief The global scope of all entities.
+struct IkarusEntityScope {
+    /// \private \brief Empty structs aren't allowed in C, so we need a dummy field.
+    short _dummy;
+};
+
+/// \brief Creates a entity scope.
+/// \return The created entity scope.
+IKA_API IkarusEntityScope ikarus_entity_scope_create();
+
+IKARUS_END_HEADER
+
+// @}