[Unpackaged] Fix taskbar glomming due to AUMID

[carlos-zamora]

Summary of the Pull Request

The bug was caused by an AUMID mismatch between the Taskbar's .lnk file and Windows Terminal. Since no AUMID was associated with the .exe, the OS automatically creates one for us. However, #20018 added an AUMID for unpackaged scenarios, so now there was a mismatch, resulting in a new taskbar entry being created.

To fix this, we check if a .lnk points to our .exe in the taskbar. There's 3 cases here:

1. no .lnk of interest exists --> set the AUMID normally
2. the .lnk carries our AUMID --> set the AUMID normally
3. the .lnk doesn't have an AUMID (aka uses the auto-resolved one) -->
   • for this launch: don't set the AUMID so that we both use the auto-resolved one
   • on next launch: set the AUMID on the .lnk and process so that they all agree

Validation Steps Performed

In unpackaged folder, move WindowsTerminal.exe to the taskbar (creates .lnk)...
✅ Double-click the .exe --> Same taskbar entry is used
✅ Double-click the .exe again --> second window goes to same taskbar entry

The first window doesn't have to close for this to work. It just * works *!

Bug introduced in #20018
Closes #20053