Move internal modules to “internal” folder.

.internal/stringSize.js

@@ -0,0 +1,18 @@
+import asciiSize from './.internal/asciiSize.js';
+import hasUnicode from './.internal/hasUnicode.js';
+import unicodeSize from './.internal/unicodeSize.js';
+
+/**
+ * Gets the number of symbols in `string`.
+ *
+ * @private
+ * @param {string} string The string to inspect.
+ * @returns {number} Returns the string size.
+ */
+function stringSize(string) {
+  return hasUnicode(string)
+    ? unicodeSize(string)
+    : asciiSize(string);
+}
+
+export default stringSize;