Add clang-format file

Also run it across the whole tree to get everything using the One True Style.
We don't yet run this in an automated fashion as it's a little slow, but
there is a snippet to do so in makeall.sh.

Shell/LineEditor.h

@@ -28,7 +28,8 @@ private:
     int m_history_cursor { 0 };
     int m_history_capacity { 100 };
 
-    enum class InputState {
+    enum class InputState
+    {
         Free,
         ExpectBracket,
         ExpectFinal,