Brief summary of issue / Description of requested feature:

We currently only have installable versions of Mudlet on https://www.mudlet.org/download, and if you want to make it portable, the steps are not so easy.

Update the packaging scripts (windows, linux, macOS) to generate a standalone-zip of Mudlet with a portable.txt file included and upload it to mudlet.org as well. Linux App images should follow portable mode standard.

Steps to reproduce the issue / Reasons for adding feature:

1. So folks can download a portable version right away, instead of having to install Mudlet normally first and then create a portable installation