Put preview-window on bottom in _fzf_search_processes

[jaminthorns]

ps outputs in a tabular format, and since the _fzf_search_processes function runs ps for a single process to generate the preview, the content of the preview is much more "horizontal" (similar to the _fzf_search_history function) since it's a single row of information.

This PR specifies that the preview-window for _fzf_search_processes should be on the bottom, 4 pixels tall, and wrapped. Since the header will always take up the first line, this gives the COMMAND 3 lines to wrap (same amount of lines that _fzf_search_history gives commands to wrap).

Before

After

[PatrickF1]

Wow that's a really great idea. We already do similar for search command history so why not for this one as well, where we might end up with very long lines! Thank you @jaminthorns !