add custom directory filter

[Xiaoy312]

for context:

[1]: D:\code\uno\framework\Uno\src\Uno.UI-Android-hotreload-vsix-only.slnf
[2]: D:\code\uno\framework\Uno\src\Uno.UI-Android-only.slnf
[3]: D:\code\uno\framework\Uno\src\Uno.UI-iOS-hotreload-vsix-only.slnf
[4]: D:\code\uno\framework\Uno\src\Uno.UI-iOS-only.slnf
[5]: D:\code\uno\framework\Uno\src\Uno.UI-macOS-only.slnf
[6]: D:\code\uno\framework\Uno\src\Uno.UI-net6-only.slnf
[7]: D:\code\uno\framework\Uno\src\Uno.UI-packages-no-net6.slnf
[8]: D:\code\uno\framework\Uno\src\Uno.UI-Skia-only.slnf
[9]: D:\code\uno\framework\Uno\src\Uno.UI-SolutionTemplates.slnf
[10]: D:\code\uno\framework\Uno\src\Uno.UI-Tools.slnf
[11]: D:\code\uno\framework\Uno\src\Uno.UI-UnitTests-only.slnf
[12]: D:\code\uno\framework\Uno\src\Uno.UI-vs4mac-iOS-only.sln
[13]: D:\code\uno\framework\Uno\src\Uno.UI-vs4mac-macOS-only.sln
[14]: D:\code\uno\framework\Uno\src\Uno.UI-vs4mac.sln
[15]: D:\code\uno\framework\Uno\src\Uno.UI-Wasm-hotreload-vsix-only.slnf
[16]: D:\code\uno\framework\Uno\src\Uno.UI-Wasm-only.slnf
[17]: D:\code\uno\framework\Uno\src\Uno.UI-Windows-only.slnf
[18]: D:\code\uno\framework\Uno\src\Uno.UI.sln

todo:

• implicit filter that is persisted to directory config
  ^ to remove that "hotreload-vsix-only" option when using filter like -f=droid
• explicit mapping where -f=key, just directly launch the mapped value
• related commands to inspect/modify the above