Define aliases just when yarn is installed

aliases/available/yarn.aliases.bash

@@ -1,6 +1,8 @@
 cite 'about-alias'
 about-alias 'yarn package manager aliases'
 
+_set_yarn_aliases() {
+	if _command_exists yarn; then
 		# Aliases
 		alias ya='yarn' # [ya]rn
 		alias yain='yarn init' # [ya]rn [in]it
@@ -48,3 +50,7 @@ alias yack='yarn check'
 		alias yals='yarn list'
 		alias yaloi='yarn login'
 		alias yaloo='yarn logout'
+	fi
+}
+
+_set_yarn_aliases