Add tasks for all the new languages

Task/String-matching/Sidef/string-matching.sidef

@@ -0,0 +1,15 @@
+var first = "abc-abcdef-abcd";
+var second = "abc";
+
+say first.begins_with(second);      #=> true
+say first.contains(second);         #=> true
+say first.ends_with(second);        #=> false
+
+# Get and print the location of the match
+say first.index(second);            #=> 0
+
+# Find multiple occurrences of a string
+var pos = -1;
+while (pos = first.index(second, pos+1) != -1) {
+    say "Match at pos: #{pos}";
+}