[QoL feature request] Custom slash commands with namespacing and arguments.

Normally when you run a custom slash command, you do not have to write out the namespace: you can simply press enter to select the highlighted command. However, if the custom command uses $ARGUMENTS, as soon as you start typing the arguments to pass to the command the autocomplete list disappears. For custom slash commands without namespacing this is not an issue, as the command is still recognized, but if the command is namespaced you have to type it out fully.

QoL improvement suggestions:

• Support automatically resolving namespace when not provided (so you don't have to manually type in the namespace for the custom command);
• Keep the command visible in the autocomplete list (so you can still read its description when you're typing out the arguments);
• Support required arguments, prompting you to enter the arguments when you try to run the command without any arguments (this is especially useful so you can still choose the command from the autocomplete list using the enter key, otherwise you have to type the command name manually).