Handle some annoyances in fish external completer (#1955)

* Escape singlequotes in fish_completer

In case the completion string has any singlequotes,
escape them before passing it to external fish command.
Else the completion will fail due to unmatched quotes.

eg. vim '/home <tab>

* fish_completer: expand path before returning completions

This fixes completions for paths which have "~".
~ expansion will no longer work after the path is enclosed
in double quotes, hence expand the path before returning the completion
so that subsequent completions will continue to work

eg. vim ~/Doc <tab> <tab>
The second completion will not work as the path will be enclosed with
double quotes after first completion

Author: scarlet-storm

cookbook/external_completers.md

@@ -20,11 +20,11 @@ This completer will use [the fish shell](https://fishshell.com/) to handle compl
 
 ```nu
 let fish_completer = {|spans|
-    fish --command $"complete '--do-complete=($spans | str join ' ')'"
+    fish --command $"complete '--do-complete=($spans | str replace "'" "\\'" | str join ' ')'"
     | from tsv --flexible --noheaders --no-infer
     | rename value description
     | update value {
-        if ($in | path exists) {$'"($in | str replace "\"" "\\\"" )"'} else {$in}
+        if ($in | path exists) {$'"($in | path expand --no-symlink | str replace --all "\"" "\\\"" )"'} else {$in}
     }
 }
 ```