[MonacoPreviewHandler][Settings] Resolve preview handler race (#16180)

* Register .markdown with the correct handler

* Fix spelling

* Move file name extensions from "expect.txt" to "excludes.txt"

* Revert "Move file name extensions from "expect.txt" to "excludes.txt""

This reverts commit 710d5a4968.
I must have misunderstood the instructions.

* Revert "Register .markdown with the correct handler"

This reverts commit 5c37b009f3.

* Work in progress

* Code ready for testing

* Update excludes.txt

* Update excludes.txt

* Update modulesRegistry.h

* Update modulesRegistry.h

For the want of an exclamation mark, a kingdom is lost!

* Update modulesRegistry.h

* Work on modulesRegistry.h per code review in 16180

Removed all previous exclusions from Monaco preview handler. Added a new exclusion: SVGZ. It's a binary file that Monaco cannot, in any meaningful way, read.

* Update expect.txt

* Update accessory files

* Disable machine-wide checks for performance reasons

tools/Verification scripts/Check preview handler registration.ps1

@@ -0,0 +1,76 @@
+#Requires -Version 7.2
+
+using namespace System.Management.Automation
+
+[CmdletBinding()]
+param()
+
+function PublicStaticVoidMain {
+  [CmdletBinding()]
+  param ()
+
+  class TypeHandlerData {
+    [String] $Name
+    [String] $CurrentUserHandler
+    [String] $MachineWideHandler
+  }
+
+  [String[]]$TypesToCheck = @(".markdown", ".mdtext", ".mdtxt", ".mdown", ".mkdn", ".mdwn", ".mkd", ".md", ".svg", ".svgz", ".pdf", ".gcode", ".stl", ".txt", ".ini")
+  $IPREVIEW_HANDLER_CLSID = '{8895b1c6-b41f-4c1c-a562-0d564250836f}'
+  $PowerToysHandlers = @{
+    '{07665729-6243-4746-95b7-79579308d1b2}' = "PowerToys PDF handler"
+    '{ddee2b8a-6807-48a6-bb20-2338174ff779}' = "PowerToys SVG handler"
+    '{ec52dea8-7c9f-4130-a77b-1737d0418507}' = "PowerToys GCode handler"
+    '{45769bcc-e8fd-42d0-947e-02beef77a1f5}' = "PowerToys Markdown handler"
+    '{afbd5a44-2520-4ae0-9224-6cfce8fe4400}' = "PowerToys Monaco fallback handler"
+    '{DC6EFB56-9CFA-464D-8880-44885D7DC193}' = "Adobe Acrobat DC"
+  }
+
+  function ResolveHandlerGUIDtoName {
+    param (
+      [Parameter(Mandatory, Position = 0)]
+      [String] $GUID
+    )
+    return $PowerToysHandlers[$GUID] ?? $GUID
+  }
+
+  function WriteMyProgress {
+    param (
+      [Parameter(Mandatory, Position=0)] [Int32] $ItemsPending,
+      [Parameter(Mandatory, Position=1)] [Int32] $ItemsTotal,
+      [switch] $Completed
+    )
+    [Int32] $PercentComplete = ($ItemsPending / $ItemsTotal) * 100
+    if ($PercentComplete -lt 1) { $PercentComplete = 1}
+    Write-Progress -Activity 'Querying Windows Registry' -Status "$ItemsPending of $ItemsTotal" -PercentComplete $PercentComplete -Completed:$Completed
+  }
+
+  $ItemsTotal = $TypesToCheck.Count * 2
+  $ItemsPending = 0
+  WriteMyProgress 0 $ItemsTotal
+
+  $CheckResults = New-Object -TypeName 'System.Collections.Generic.List[TypeHandlerData]'
+  foreach ($item in $TypesToCheck) {
+    $CurrentUserGUID = Get-ItemPropertyValue -Path "HKCU://Software/Classes/$item/shellex/$IPREVIEW_HANDLER_CLSID" -Name '(default)' -ErrorAction SilentlyContinue
+    $ItemsPending += 1
+    WriteMyProgress $ItemsPending $ItemsTotal
+
+    $MachineWideGUID = "Didn't check"
+    # $MachineWideGUID = Get-ItemPropertyValue -Path "HKLM://Software/Classes/$item/shellex/$IPREVIEW_HANDLER_CLSID" -Name '(default)' -ErrorAction SilentlyContinue
+    $ItemsPending += 1
+    WriteMyProgress $ItemsPending $ItemsTotal
+
+    $temp = New-Object -TypeName TypeHandlerData -Property @{
+      Name               = $item
+      CurrentUserHandler = ($null -eq $CurrentUserGUID) ? "Nothing" : (ResolveHandlerGUIDtoName ($CurrentUserGUID))
+      MachineWideHandler = ($null -eq $MachineWideGUID) ? "Nothing" : (ResolveHandlerGUIDtoName ($MachineWideGUID))
+    }
+    $CheckResults.Add($temp)
+
+    Clear-Variable 'CurrentUserGUID', 'MachineWideGUID'
+  }
+  WriteMyProgress $ItemsPending $ItemsTotal -Completed
+  $CheckResults | Format-Table -AutoSize
+}
+
+PublicStaticVoidMain