Add pangram exercise

pangram/example.sml

@@ -0,0 +1,7 @@
+fun isPangram (sentence: string): bool =
+  let
+    val alphabet = String.explode "qwertyuiopasdfghjklzxcvbnm"
+    val chars = map Char.toLower (String.explode sentence)
+  in
+    List.all (fn letter => List.exists (fn char => char = letter) chars) alphabet
+  end

pangram/pangram.sml

@@ -0,0 +1,2 @@
+fun isPangram (sentence: string): bool =
+  raise Fail "isPangram is not implemented"

pangram/test_pangram.sml

@@ -0,0 +1,21 @@
+use "example.sml";
+
+val test_cases = [
+  ("", false),
+  ("The Quick Brown Fox Jumps Over The Lazy Dog", true),
+  ("the quick brown fox jumps over the lazy dog", true),
+  ("a quick movement of the enemy will jeopardize five gunboats", false),
+  ("the quick brown fish jumps over the lazy dog", false),
+  ("the_quick_brown_fox_jumps_over_the_lazy_dog", true),
+  ("the 1 quick brown fox jumps over the 2 lazy dogs", true),
+  ("7h3 qu1ck brown fox jumps ov3r 7h3 lazy dog", false),
+  ("\"Five quacking Zephyrs jolt my wax bed.\"", true)
+  (*"Victor jagt zwölf Boxkämpfer quer über den großen Sylter Deich.", true*)
+  (*"Широкая электрификация южных губерний даст мощный толчок подъёму сельского хозяйства.", false*)
+];
+
+fun run_tests [] = []
+  | run_tests ((sentence, expected) :: ts) =
+       (isPangram sentence = expected) :: run_tests ts
+
+val allTestsPass = List.foldl (fn (x, y) => x andalso y) true (run_tests test_cases)