feat: Add ShadowContainer Sample Page & TextBox Shadow Style (backport #849)

[mergify]

This is an automatic backport of pull request #849 done by Mergify.
Cherry-pick of 6fdfd38 has failed:

On branch mergify/bp/legacy/4x/pr-849
Your branch is up to date with 'origin/legacy/4x'.

You are currently cherry-picking commit 6fdfd38.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   Uno.Gallery/Uno.Gallery.Shared/Controls/ContentPageLayout.cs
	modified:   Uno.Gallery/Uno.Gallery.Shared/Views/GeneralPages/CupertinoPalettePage.xaml
	modified:   Uno.Gallery/Uno.Gallery.Shared/Views/GeneralPages/CupertinoPalettePage.xaml.cs
	modified:   Uno.Gallery/Uno.Gallery.Shared/Views/GeneralPages/FluentPalettePage.xaml
	modified:   Uno.Gallery/Uno.Gallery.Shared/Views/GeneralPages/MaterialPalettePage.xaml
	modified:   Uno.Gallery/Uno.Gallery.Shared/Views/GeneralPages/MaterialPalettePage.xaml.cs
	new file:   Uno.Gallery/Uno.Gallery.Shared/Views/NeumorphicColorsOverride.xaml
	new file:   Uno.Gallery/Uno.Gallery.Shared/Views/SamplePages/ShadowContainerSamplePage.xaml
	new file:   Uno.Gallery/Uno.Gallery.Shared/Views/SamplePages/ShadowContainerSamplePage.xaml.cs
	new file:   Uno.Gallery/Uno.Gallery.Shared/Views/Styles/NeumorphicStyles.xaml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   Uno.Gallery/Uno.Gallery.Mobile/Uno.Gallery.Mobile.csproj
	both modified:   Uno.Gallery/Uno.Gallery.Skia.Gtk/Uno.Gallery.Skia.Gtk.csproj
	both modified:   Uno.Gallery/Uno.Gallery.Wasm/Uno.Gallery.Wasm.csproj
	both modified:   Uno.Gallery/Uno.Gallery.Windows/Uno.Gallery.Windows.csproj

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[agneszitte]

Will need to wait after unoplatform/uno.toolkit.ui#703 for Uno.Toolkit.Skia.WinUI package