[FEATURE REQUEST] Install the "winget-extras" custom Winget source by default.

### Please confirm these before moving forward.

- [x] I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
- [x] This proposal is a completely new feature. If you want to suggest an improvement or an enhancement, please use [this template](https://github.com/Devolutions/UniGetUI/issues/new?labels=enhancement&projects=&template=enhancement-improvement.yml&title=%5BENHANCEMENT%5D+%28Enter+your+description+here%29).

### Describe the new feature

There's already precedence for various non-main Scoop buckets being added to UniGetUI by default, so adding [winget-extras](https://github.com/pl4nty/winget-extras) as a non-main Winget source would've been pretty good to do.

### Describe how this new feature could help users

Due to the ongoing slow but steady collapse of adding new packages to the main Winget source; see:

* https://github.com/microsoft/winget-pkgs/discussions/333858.

Then an increasing number of Winget packages are likely to be found only in winget-extras and not in the main source. This includes, but is not limited to:
* Famatech Advanced Port Scanner
* Fonts in general
* Game Jolt
* HopToDesk
* Maxon Cinebench
* Microsoft OneNote
* Microsoft Outlook
* ModMii
* MozillaBuild
* NirCmd
* NVIDIA GeForce drivers (both the standard channel and the 7x0-10x0 cards' LTSC channel)
* Preview builds of App Installer
* Standalone x86 versions of .NET Runtime and .NET Desktop Runtime
* vcpkg

No file or test entry point is named. Start by tracing how UniGetUI registers existing non-main Scoop buckets and Winget sources, then inspect the winget-extras source metadata; done means the source is installed by default and its packages appear without manual setup.

Written by the indexing model from the issue text.