Initialize _fisher_plugins using fish_plugins file (jorgebucaran#741)

This change is for users who commit fish_plugins and plugin sources but
omit fish_variables. On a system where Fisher's universal variables are
unset, most Fisher commands were not working out-of-the-box:

* `fisher install <plugin>` - installs <plugin>, rewrites fish_plugins file with only <plugin>
                            - if <plugin> is in fish_plugins, same behavior as `fisher update`
* `fisher remove  <plugin>` - fails with error: Plugin not installed "<plugin>"
* `fisher update  <plugin>` - fails with error: Plugin not installed "<plugin>"
* `fisher update`           - fails with error about conflicting files, deletes fish_plugins file
* `fisher list   [<regex>]` - returns nothing

As of this commit all Fisher commands work except for `fisher remove`;
Fisher still has no way of knowing which files to remove absent the
universal variable that tracks the files associated to a plugin.

It may make sense to reject calls like `fisher remove <plugin>` if the
`_fisher_<plugin>_files` universal variable is missing. Fisher could
suggest the user run `fisher update` and try again.

functions/fisher.fish

@@ -2,6 +2,8 @@ function fisher --argument-names cmd --description "A plugin manager for Fish"
     set --query fisher_path || set --local fisher_path $__fish_config_dir
     set --local fisher_version 4.4.3
     set --local fish_plugins $__fish_config_dir/fish_plugins
+    test -e $fish_plugins && set --local file_plugins (string match --regex -- '^[^\s]+$' <$fish_plugins)
+    set --query _fisher_plugins || set --universal _fisher_plugins $file_plugins
 
     switch "$cmd"
         case -v --version
@@ -29,8 +31,6 @@ function fisher --argument-names cmd --description "A plugin manager for Fish"
             set --local old_plugins $_fisher_plugins
             set --local new_plugins
 
-            test -e $fish_plugins && set --local file_plugins (string match --regex -- '^[^\s]+$' <$fish_plugins)
-
             if ! set --query argv[2]
                 if test "$cmd" != update
                     echo "fisher: Not enough arguments for command: \"$cmd\"" >&2 && return 1