Deprecate legacy v1/v2/v3 C++ API; make v3 C the mainline

Move tiny_gltf.h/.cc, tinygltf_json.h, json.hpp, stb_image*, loader_example.cc,
examples/, wasm/, experimental/, legacy tests and fuzzers under attic/.

TinyGLTF v3 C (tiny_gltf_v3.h/.c + tinygltf_json_c.h) is now the mainline:
- CMakeLists.txt: v3 C tests only, installs only v3 files
- tests/Makefile and Makefile: build/run the v3 C testers
- test_runner.py: verifies v3 C tester output only (no v1 ground truth)
- CI workflows: build and test v3 C only (GCC/Clang/MSVC/Meson/sanitizers)
- README: v3 C quick start, testing, licenses; legacy moved to attic/

attic/examples/basic/window.cpp

@@ -0,0 +1,27 @@
+#include "window.h"
+
+
+
+Window::Window(int x, int y, const char* title)
+{
+	GLFWwindow* window = glfwCreateWindow(x, y, title, NULL, NULL);
+	this->window = window;
+}
+
+
+Window::~Window()
+{
+	glfwDestroyWindow(this->window);
+}
+
+void Window::Resize() 
+{
+	GLint w, h;
+	glfwGetWindowSize(this->window, &w, &h);
+	glViewport(0, 0, w, h);
+}
+
+int Window::Close()
+{
+	return glfwWindowShouldClose(this->window);
+}