feat(controls): Implement SpinButtonPlacementMode.Compact for NumberBox

[chucker]

NumberBox already had a SpinButtonPlacementMode.Compact; however, it previously had the same effect as Hidden. This adds a style to NumberBox that will:

• add a button with an up/down symbol:

• when clicked, pop out a flyout, with buttons to increment/decrement:

It also expands Gallery's NumberBoxPage to let users try out the three variants.

Pull request type

Please check the type of change your PR introduces:

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

Issue Number: fixes #1673

What is the new behavior?

SpinButtonPlacementMode.Compact now has its own look/behavior.

Other information

The result roughly matches the look+feel in WinUI 3 Gallery 2.9.0.0:

Minor differences:

• there's no Flyout drop shadow (see Flyout should have a drop shadow #1708)
• the buttons should probably have the "subtle" style, which I don't believe WPF UI implements; from the WinUI gallery:

[stanlrt]

Thanks for doing it, in the end i never found the time