Refactored libimhex to and includes to better represent it as library

plugins/libimhex/include/hex/plugin.hpp

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <glad/glad.h>
+#include <imgui.h>
+
+#include <hex.hpp>
+#include <hex/api/content_registry.hpp>
+#include <hex/views/view.hpp>
+#include <hex/providers/provider.hpp>
+#include <hex/helpers/shared_data.hpp>
+
+#define IMHEX_PLUGIN_SETUP      namespace hex::plugin::internal {               \
+                                    void initializePlugin();                    \
+                                }                                               \
+                                void hex::plugin::internal::initializePlugin()