ocaml: phase 5.1 number_words.ml baseline (letter count of 1..19 spelled out = 106)

19-arm match returning the English word for each number 1..19, then
sum String.length:

  let number_to_words n =
    match n with
    | 1 -> 'one' | 2 -> 'two' | ... | 19 -> 'nineteen'
    | _ -> ''

  total_letters 19 = 36 + 70 = 106
                    (1-9)  (10-19)

Real PE17 covers 1..1000 (answer 21124) but needs more elaborate
number-to-words logic (compounds, 'and', 'thousand'). 1..19 keeps
the program small while exercising literal-pattern match dispatch
on many arms.

128 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -86,6 +86,7 @@
   "module_use.ml": 3,
   "monotonic.ml": 4,
   "newton_sqrt.ml": 1414,
+  "number_words.ml": 106,
   "mutable_record.ml": 10,
   "option_match.ml": 5,
   "palindrome.ml": 4,