Enhancement: new builtin shortcuts: active_office_file

[1208nn]

Fixes #2936

Add new builtin shortcut {active_office_file} to quickly access the active office file.

• Add a new method GetActiveOfficeFilePath in Flow.Launcher.Infrastructure/Helper.cs to retrieve the active office file path.
• Add a new builtin shortcut {active_office_file} in Flow.Launcher.Infrastructure/UserSettings/Settings.cs to use the GetActiveOfficeFilePath method.
• Update the BuiltinShortcuts list in Flow.Launcher.Infrastructure/UserSettings/Settings.cs to include the new {active_office_file} shortcut.

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