Move internal modules to “internal” folder.

.internal/charsStartIndex.js

@@ -0,0 +1,20 @@
+import baseIndexOf from './.internal/baseIndexOf.js';
+
+/**
+ * Used by `trim` and `trimStart` to get the index of the first string symbol
+ * that is not found in the character symbols.
+ *
+ * @private
+ * @param {Array} strSymbols The string symbols to inspect.
+ * @param {Array} chrSymbols The character symbols to find.
+ * @returns {number} Returns the index of the first unmatched string symbol.
+ */
+function charsStartIndex(strSymbols, chrSymbols) {
+  let index = -1;
+  const length = strSymbols.length;
+
+  while (++index < length && baseIndexOf(chrSymbols, strSymbols[index], 0) > -1) {}
+  return index;
+}
+
+export default charsStartIndex;