perf: optimize collapseLeadingSlashes function

[Phillip9587]

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.

[bjohansebas]

I'm not sure this is a performance improvement, but I still think it's a good change.

[rmhaiderali]

What do you guys think of this?

function collapseLeadingSlashes(str) {
  return str.replace(/^\/+/, "/")
}