Support Zed "debug" operator in Zui

[philrz]

At the time this issue is being opened, Zui is at commit aa15e3e.

brimdata/super#5196 added the debug operator to the Zed language. Similar to how debug sends its output to stderr when used with the Zed CLI tools, we've discussed having the Zui app send debug output somewhere prominent in the app, such as to a separate tab/window.

Right now debug only sends its output to a single named channel called debug with the regular query output appearing on a channel called main.