Move FilePicker::render from Component impl to normal impl

Merges the code for the Picker and FilePicker into a single Picker that
can show a file preview if a preview callback is provided. This change
was mainly made to facilitate refactoring out a simple skeleton of a
picker that does not do any filtering to be reused in a normal Picker
and a DynamicPicker (see #5714; in particular [mikes-comment] and
[gokuls-comment]).

The crux of the issue is that a picker maintains a list of predefined
options (eg. list of files in the directory) and (re-)filters them every
time the picker prompt changes, while a dynamic picker (eg. interactive
global search, #4687) recalculates the full list of options on every
prompt change. Using a filtering picker to drive a dynamic picker hence
does duplicate work of filtering thousands of matches for no reason. It
could also cause problems like interfering with the regex pattern in the
global search.

I tried to directly extract a PickerBase to be reused in Picker and
FilePicker and DynamicPicker, but the problem is that DynamicPicker is
actually a DynamicFilePicker (i.e. it can preview file contents) which
means we would need PickerBase, Picker, FilePicker, DynamicPicker and
DynamicFilePicker and then another way of sharing the previewing code
between a FilePicker and a DynamicFilePicker. By merging Picker and
FilePicker into Picker, we only need PickerBase, Picker and
DynamicPicker.

[gokuls-comment]: https://github.com/helix-editor/helix/issues/5714#issuecomment-1410949578
[mikes-comment]: https://github.com/helix-editor/helix/issues/5714#issuecomment-1407451963

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