XD-1585: Completions after a named channel

Adds completions for cases like
```
queue:foo > <TAB>
queue:foo > splu<TAB>
```

with one caveat: processors will not be part of proposals for the second case (XD-1830)
Do we need to add other samples here: `topic:foo`, `tap:stream:foo`, `tap:queue:foo`, `tap:job:foo` and `tap:topic:foo` ?