Add tasks for all the new languages

Task/Substring/Nim/substring.nim

@@ -0,0 +1,26 @@
+import strutils
+
+let
+  s = "abcdefgh"
+  n = 2
+  m = 3
+  c = 'd'
+  cs = "cd"
+var i = 0
+
+# starting from n=2 characters in and m=3 in length
+echo s[n-1 .. n+m-2]
+
+# starting from n characters in, up to the end of the string
+echo s[n-1 .. s.high]
+
+# whole string minus last character:
+echo s[0 .. <s.high]
+
+# starting from a known character c='d'within the string and of m length
+i = s.find(c)
+echo s[i .. <i+m]
+
+# starting from a known substring cs="cd" within the string and of m length
+i = s.find(cs)
+echo s[i .. <i+m]