chore: add commonly used files

* add makefile
* add c++ skeleton

Signed-off-by: Matej Focko <mfocko@redhat.com>

.common/makefile

@@ -0,0 +1,8 @@
+CXX=clang++
+CXXFLAGS=-std=c++20 -Wall -Wextra
+
+main: format main.cpp
+	$(CXX) $(CXXFLAGS) -g main.cpp -o main
+
+format:
+	clang-format -i -style=google main.cpp

.common/skeleton.cpp

@@ -0,0 +1,14 @@
+#include <iostream>
+
+namespace {
+
+void solve() {
+  // TODO
+}
+
+}  // namespace
+
+int main(void) {
+  solve();
+  return 0;
+}