add: millennium-uwp-library-sync submodule

[xwxfox]

UWP & XBOX Library Sync

Allows you to automatically add & manage your UWP & XBOX (app) games to Steam - and keep them in sync whenever you relaunch steam

Task Checklist

Developer

• I am the original author or an authorized maintainer of this plugin.
• I have complied with all license requirements for the libraries used, including providing appropriate notices where necessary.
• My plugin is fully open source and does not depend on any external paid services, except for widely trusted and well-known platforms. Additionally, neither I nor anyone associated with me profits from any such services.

Plugin Functionality

• I have tested the plugin on both the Stable and Beta Steam update channels.

Tested on Stable - but baseline Millennium doesnt seem to work on Beta (at least for me)

• My plugin is unique, or provides additional or alternative functionality to plugins already on the store.

Backend Configuration

• Yes: I use a standard Millennium python backend in my plugin.
• Yes: I use custom binaries that or rely on other FOSS projects that aren't written directly using Millennium's python backend.

i use powershell & cmd for some internal stuff like getting the UWP apps, and for compiling the tiny .NET launcher we launch UWP apps with so steam can keep track of it.

Community Contribution

• I have tested and left feedback on two other plugin pull requests.
• I have added links to those feedback comments in this PR.

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Testing Instructions

• Verified by a third party on Steam Client Stable.
• Verified by a third party on Steam Client Beta.