Handle installation for 'fish' shell (yarnpkg#690)

* Handle installation for 'fish' shell

'fish' is a popular shell. This commit adds support for proper detection and installation of yarn in 'fish' shell. If the detected shell is 'fish', add path to ~/.yarn/bin in $HOME/.config/fish/config.fish file.

* Use a more modern way of setting $PATH in fish

Set `~/.yarn/bin` in `$fish_user_paths` instead of setting it in `~/.config/fish/config.fish` file.

See [suggestion from fish documentation](fish-shell/fish-shell#527 (comment)) for more reference.

* Run command to set $fish_user_paths instead of evaluating a string.

scripts/install-latest.sh

@@ -31,7 +31,11 @@ yarn_link() {
     command printf "${SOURCE_STR}"
   else
     if ! grep -q 'yarn' "$YARN_PROFILE"; then
-      command printf "$SOURCE_STR" >> "$YARN_PROFILE"
+      if [[ $YARN_PROFILE == *"fish"* ]]; then
+        command fish -c 'set -U fish_user_paths $fish_user_paths ~/.yarn/bin'
+      else
+        command printf "$SOURCE_STR" >> "$YARN_PROFILE"
+      fi
     fi
 
     printf "$cyan> We've added the following to your $YARN_PROFILE\n"
@@ -61,6 +65,8 @@ yarn_detect_profile() {
     fi
   elif [ "$SHELLTYPE" = "zsh" ]; then
     DETECTED_PROFILE="$HOME/.zshrc"
+  elif [ "$SHELLTYPE" = "fish" ]; then
+    DETECTED_PROFILE="$HOME/.config/fish/config.fish"
   fi
 
   if [ -z "$DETECTED_PROFILE" ]; then
@@ -72,6 +78,8 @@ yarn_detect_profile() {
       DETECTED_PROFILE="$HOME/.bash_profile"
     elif [ -f "$HOME/.zshrc" ]; then
       DETECTED_PROFILE="$HOME/.zshrc"
+    elif [ -f "$HOME/.config/fish/config.fish" ]; then
+      DETECTED_PROFILE="$HOME/.config/fish/config.fish"
     fi
   fi