Add tasks for all the new languages

Task/Execute-a-system-command/Sidef/execute-a-system-command.sidef

@@ -0,0 +1,10 @@
+# Pipe in read-only mode
+%p(ls).open_r.each { |line|
+    print line;
+};
+
+var str1 = `ls`;         # backtick: returns a string
+var str2 = %x(ls);       # ditto, alternative syntax
+
+Sys.system('ls');   # system: executes a command and prints the result
+Sys.exec('ls');     # replaces current process with another