Add UTF-8 support to the JavaScript API

This adds an optional argument to generated getters for string fields to
specify the encoding type and overloads the createString() function. It's
now possible to use either JavaScript UTF-16 string objects or C-style
UTF-8 byte arrays (Uint8Array) for string data.

tests/unicode_test.json

@@ -1,5 +1,13 @@
 {
   "name": "unicode_test",
+  "testarrayoftables": [
+    { "name": "Цлїςσδε" },
+    { "name": "ﾌﾑｱﾑｶﾓｹﾓ" },
+    { "name": "フムヤムカモケモ" },
+    { "name": "㊀㊁㊂㊃㊄" },
+    { "name": "☳☶☲" },
+    { "name": "𡇙𝌆" }
+  ],
   "testarrayofstring": [
     "Цлїςσδε",
     "ﾌﾑｱﾑｶﾓｹﾓ",