Auto-format our XAML files and enforce in CI (#9589)

This adds [`XamlStyler.Console`] to our solution, and calls it when we
format the code, to also format
our .xaml files. 
* `XamlStyler.Console` is a dotnet tool so it needs to be restored with
  `dotnet tool restore`
* I've added a set of rules to approximately follow [@cmaneu's XAML guidelines].
  Those guidelines also recommend things based on the code-behind, which
  this tool can't figure out, but also _don't matter that much_.
* There's an extra step to strip BOMs from the output, since Xaml Styler
  adds a BOM by default. Some had them before and others didn't. BOMs
  have been nothing but trouble though.

[`XamlStyler.Console`]: https://github.com/Xavalon/XamlStyler
[@cmaneu's XAML guidelines]: https://github.com/cmaneu/xaml-coding-guidelines

build/scripts/Invoke-FormattingCheck.ps1

@@ -3,12 +3,24 @@
 # Checks for code formatting errors. Will throw exception if any are found.
 function Invoke-CheckBadCodeFormatting() {
     Import-Module ./tools/OpenConsole.psm1
-    Invoke-CodeFormat
+
+    # Don't run the XAML formatter in this step - even if it changes nothing,
+    # it'll still touch all the .xaml files.
+    Invoke-CodeFormat -IgnoreXaml
+
     # returns a non-zero exit code if there are any diffs in the tracked files in the repo
     git diff-index --quiet HEAD --
     if ($lastExitCode -eq 1) {
+
+        # Write the list of files that need updating to the log
+        git diff-index --name-only HEAD
+
         throw "code formatting bad, run Invoke-CodeFormat on branch"
     }
+
+    # Manually check the formatting of our .xaml files, without touching them.
+    Verify-XamlFormat
+
 }
 
 Invoke-CheckBadCodeFormatting