project: add .clangd, where we set -Wno-c2y-extensions

We make use of __COUNTER__, which recent versions of clang emit a
warning for (unless you compile with -std=c2y).

Our meson.build already handles this for the actual build, but if your
build is using gcc, -Wno-c2y-extensions isn't added to the build
rules (since gcc doesn't support the flag).

.clangd

@@ -0,0 +1,5 @@
+# -*- yaml -*-
+
+CompileFlags:
+  Add:
+    - -Wno-c2y-extensions