feat: Make `__fundle_init` return 1 when a plugin is not installed

[SirWrexes]

After a quick look through fundle.fish, I noticed that __fundle_init's return value isn't used anywhere. In a way, it makes sens because it is not a function that fundle itself relies on the output of to determine anything. For a user, though, this makes scripting the installation of plugins slightly more tedious.

This PR changes that. It's a very simple change, but still a great quality of life improvement IMO.

As for a concrete, real-life example, my plugin initialisation file contains this: (Skip to the last if statement if you only want to see the change brought by this PR)

# Automatically install fundle
if not functions -q fundle;
    eval (curl -sfL https://git.io/fundle-install);
end

# Easily access your plugin folder
set -Ux fish_plugin_dir (realpath $__fish_config_dir/fundle)
# And plugin list
set -Ux fish_plugins (realpath $__fish_config_dir/fish_plugins)

# Create the directory if necessary
if ! test -d $fish_plugin_dir
  mkdir -p $fish_plugin_dir
end

# Read the plugin list and store it
set -l __plugins (
  string split0 -- (
    # Filter out commented lines
    string match -r '^\s*[^#].+$' -- (cat $fish_plugins)
  )
)

# Enable found plugins
for p in $__plugins
  fundle plugin $p
end

# Try initialising plugins.
# If any are missing, fundle will output a message, and stay silent otherwise.
# This is what we test to determine if we need to run the install command.
if string match -qr 'not installed' -- (string collect -- (fundle init))
    set_color brgreen
      echo "Populating your plugin directory ($fish_plugin_dir)..."
    set_color normal
      fundle install | string match -er 'Installing'
    set_color brgreen
      echo "Done !"
    set_color normal
    exec fish
end

Note how the last if's test is clearly suboptimal, whereas when __fundle_init returns 1 on uninstalled plugins, you could change it to

# Try initialising plugins.
# If any are missing, install them and reload the shell.
if not fundle init >/dev/null
    # ...
end

[danhper]

Thanks and sorry for the delay