[ide] provide information about abstract `from` in toplevel completion expected type info

This would be handy for cases like this:

```haxe
abstract Callback(T->Void) from T->Void {}

function main() {
var cb:Callback = |
}
```

If haxe language server had the info about implicit abstract conversions, it could provide "generate anonymous function" snippet completion. I assume the same would work for structure types.