refactor: restructure project to build a library for each book chapter

chapters/chapter_02/include/constexp.h

@@ -0,0 +1,5 @@
+#pragma once
+
+namespace constexp {
+void test();
+};

chapters/chapter_02/include/ifswitch.h

@@ -0,0 +1,7 @@
+#pragma once
+
+namespace ifswitch {
+
+void test();
+
+};

chapters/chapter_02/include/initlist.h

@@ -0,0 +1,5 @@
+#pragma once
+
+namespace initlist {
+void test();
+};

chapters/chapter_02/include/null.h

@@ -0,0 +1,5 @@
+#pragma once
+
+namespace null {
+void test();
+};