Move internal modules to “internal” folder.

.internal/createCaseFirst.js

@@ -0,0 +1,33 @@
+import castSlice from './.internal/castSlice.js';
+import hasUnicode from './.internal/hasUnicode.js';
+import stringToArray from './.internal/stringToArray.js';
+import toString from './toString.js';
+
+/**
+ * Creates a function like `lowerFirst`.
+ *
+ * @private
+ * @param {string} methodName The name of the `String` case method to use.
+ * @returns {Function} Returns the new case function.
+ */
+function createCaseFirst(methodName) {
+  return string => {
+    string = toString(string);
+
+    const strSymbols = hasUnicode(string)
+      ? stringToArray(string)
+      : undefined;
+
+    const chr = strSymbols
+      ? strSymbols[0]
+      : string.charAt(0);
+
+    const trailing = strSymbols
+      ? castSlice(strSymbols, 1).join('')
+      : string.slice(1);
+
+    return chr[methodName]() + trailing;
+  };
+}
+
+export default createCaseFirst;