Enable -Wimplicit-fallthrough.

Kernel/VirtualConsole.cpp

@@ -417,14 +417,14 @@ void VirtualConsole::on_char(byte ch)
             return;
         }
         m_escape_state = ExpectIntermediate;
-        // fall through
+        [[fallthrough]];
     case ExpectIntermediate:
         if (is_valid_intermediate_character(ch)) {
             m_intermediates.append(ch);
             return;
         }
         m_escape_state = ExpectFinal;
-        // fall through
+        [[fallthrough]];
     case ExpectFinal:
         if (is_valid_final_character(ch)) {
             m_escape_state = Normal;