* Copy completion filter to custom completions
* Remove filter function from completer
This function was a no-op for FileCompletion and CommandCompletion.
Flag- and VariableCompletion just filters with `starts_with` which
happens in both completers anyway and should therefore also be a no-op.
The remaining use case in CustomCompletion was moved into the
CustomCompletion source file.
Filtering should probably happen immediately while fetching completions
to avoid unnecessary memory allocations.
* Add get_sort_by() to Completer trait
* Remove CompletionOptions from Completer::fetch()
* Fix clippy lints
* Apply Completer changes to DotNuCompletion
