core: Add clang-tidy (#8664)

This adds a .clang-tidy file for us.

It's not a strict requirement to be compliant, but I tuned it to be alright.

src/xwayland/XWM.hpp

@@ -58,8 +58,8 @@ struct SXSelection {
 
 class CXCBConnection {
   public:
-    CXCBConnection(int fd) {
-        connection = xcb_connect_to_fd(fd, nullptr);
+    CXCBConnection(int fd) : connection{xcb_connect_to_fd(fd, nullptr)} {
+        ;
     }
 
     ~CXCBConnection() {