More useful fn detail in completion

Detail should be rendered as shtort one line, just dumping fn header
there is not useful, despite the fact that TS does this.

The fact that this is a function should be indicated by the icon, the
same goes for pub/const/async etc qualitfiers

name is already present in the lable (and arg list should be a part of
that, as in idea)

But the return type is the small genuinerlly useful bit of info we can show here

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