Add UNIX file type bits when only permissions are given

[maximilliangrand]

Bug

Setting unixPermissions to a bare permission value - the "755" form shown in JSZip's own docs - writes an entry whose UNIX mode has no file-type bits:

const zip = new JSZip();
zip.file("script.sh", "#!/bin/bash", { unixPermissions: "755" });
const buf = await zip.generateAsync({ type: "nodebuffer", platform: "UNIX" });
// zipinfo shows:  ?rwxr-xr-x  ... script.sh   (unknown file type)

Cause

generateUnixExternalFileAttr() only fills in the file-type bits (S_IFMT) in its default branch, when no permissions are supplied (0100664/040775). A user-supplied value is written through unchanged, so a bare 0755 ends up with file-type 0. The default path and the user path therefore produce different-quality output.

Fix

When permissions are given but the S_IFMT bits are absent, add the regular-file / directory type, matching the default JSZip already uses. A value that already carries a file type (e.g. a 0120777 symlink, per #428) is left untouched.

Evidence

zipinfo on the same input after the fix: -rwxr-xr-x script.sh, drwxr-xr-x dir/, lrwxrwxrwx link (symlink type preserved). Added a round-trip test in permissions.js; it fails on main (755 vs 100755) and passes with the fix. Lint clean.