New Client Side Command: @Clear to clean up the output window

## Add a new Client Side Command @clear to clean up the output window ##

As there is the client command @echo to print a line in the output window, it is certainly just as useful to have a @clear command that cleans up the output window.

In this way, you can automate the cleanup of the output before running a long SQL script or before invoking a procedure or function that writes a lot of output.