ensure that there is at least one leading slash at the beginning.

import { assertEquals } from "jsr:@std/assert"

// aliasing our functions for brevity
const eq = assertEquals, fn = ensureStartSlash

eq(fn("a/b.zip"), "/a/b.zip")
eq(fn(".///../a/b.zip/"), "/.///../a/b.zip/")
eq(fn("///../a/b.zip/"), "///../a/b.zip/")
eq(fn("file:///a/b.zip//c.txt"), "/file:///a/b.zip//c.txt")