Warning AVLN2208: Item container in the data template

[maxkatz6]

What does the pull request do?

It's a fairly common mistake to put item container inside of data template.
Typical example can be found even in this repository:

Avalonia/src/Avalonia.Diagnostics/Diagnostics/Views/EventsPageView.xaml

Lines 74 to 80 in 151d97a

	<ListBox Name="EventsList" ItemsSource="{Binding RecordedEvents}"
	SelectedItem="{Binding SelectedEvent, Mode=TwoWay}">
	<ListBox.ItemTemplate>
	<DataTemplate>
	<ListBoxItem Classes.handled="{Binding IsHandled}">
	<Grid ColumnDefinitions="Auto,Auto,Auto,*,Auto">

The problem with this code is that DataTemplate does not affect how container itself is defined. Instead, it will create a dumb ListBoxItem inside of the proper ListBoxItem. Making it worse, styles are active for both ListBoxItems, but only outer one can be actually selected.

Example of how this code looks in the elements tree:

This problem is not limited by ListBoxItem, but by any items control. I often see this mistake with MenuItem and TabControl.

What is the updated/expected behavior with this PR?

New XAML analyzer is added to XamlX compiler.
Code: "AVLN2208" (can be configured as an error via project settings or editorconfig, or disabled).
Message example (actual text depends on the context):

Unexpected 'TabItem' inside of 'TabControl.DataTemplates'. 'TabControl.DataTemplates' defines template of the container content, not the container itself.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

By default, this is a warning. No user code will have these errors, unless these projects have all warnings enabled as errors - but these projects typically expect new errors to be found early.

Fixed issues

Contributes towards #13707

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054805-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]