Fix escaping fzf query

[justbispo]

I noticed that when trying to get fzf completion for directories that had backslashes to escape characters, it wouldn't work.
string trim only removes the backslashes from the beginning and the end of the string, not the middle (which it didn't even do because the correct syntax is '\\', not '\'').

Fish has the command string unescape, seems to be the correct one for this context.

In commit cfe17fb, the variable fifc_fzf_query changes to fifc_query in its declaration and in fzf_cmd, but not in this line:

fifc/functions/_fifc.fish

Line 29 in cfe17fb

set fifc_fzf_query (string trim --chars '\'' -- "$fifc_fzf_query")

which means the trim stopped applying.