Refactor collapseLeadingSlashes function for simplicity and readability

Replaced the old implementation of collapseLeadingSlashes, which iterated over each character, with a more concise and efficient version using a regular expression. The new version uses 
eplace() to collapse leading slashes into a single slash and handles empty strings by returning '/'. This update improves code readability and leverages modern JavaScript string methods.

Author: Ayoub-Mabrouk

index.js

@@ -127,6 +127,9 @@ function serveStatic (root, options) {
 
 /**
  * Collapse all leading slashes into a single slash
+ *
+ * @param {string} str
+ * @returns {string}
  * @private
  */