slash command suggestions

[uhoreg]

• when you type a slash command, the autocomplete box will pop up. When you start typing an invalid slash command, it would be nice to have immediate feedback that the command is invalid instead of waiting until hitting enter (somewhat related to Allow sending messages with leading slash in Riot #4630)
• when typing a slash command, it would be nice if the autocomplete box stuck around after you typed the command name, since the autocomplete box shows the syntax of the command

[lampholder]

My first reaction was "isn't this a duplicate of something; maybe #5069".

But it's not :)

[t3chguy]

I've ticked off #2 with the above PR :D

[t3chguy]

• similarly to point one, it'd be nice if the command could warn you about malformed arguments before you submit the command, much a stretch goal