perf: optimize collapseLeadingSlashes function

[Phillip9587]

This pull request refactors the collapseLeadingSlashes function in index.js to improve readability and performance by simplifying the loop and replacing substr with slice.

Code readability and performance improvements:

• index.js: Simplified the collapseLeadingSlashes function by combining the loop condition and increment into a single line, and replaced the deprecated substr method with the more modern slice method for better compatibility.