feat: ✨ Customize workspace folder icons

[PKief]

Description

This PR implements the feature of customizing workspace folder icons:

It makes use of all existing features including color changing and cloning. So it's possible to give every icon an individual touch if needed. Both user settings for local customization as well as preconfigured via "folderIcons.ts" is supported.

I made the experience that workspace folder are usually very individual. That's why we should keep the default folder icon here. If people want to change the behavior they can adjust it in their user settings.

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Todos:

• Extend the configuration
• Add the rootFolderNames and rootFolderNamesExpanded to the manifest
• Register new command for root folder color change
• Add translations
• Include cloning mechanism of folder icons
• Adjust documentation (Contributing.md and Readme.md)
• Adjust tests
• Test locally to double check functionality
• Check behavior of detecting changes while activating the extension for the first time

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