Implement lazy slash commands

[diamondburned]

The current autocompletion API is too weak to implement actual slash commands in the input. It's also really hard to fit rich text boxes within the TextInput, and handling text bounds is very complicated.

This issue proposes to use the autocompletion API just for the initial / prefix and autocompletion, after which a new dialog will be popped up with a list of arguments to be used as buttons rather than emulating command-like behavior. In the future, we can consider replacing this method, but for now, it's better to at least have something.

The command dialog can look like this: