[FluentList] Set environment variable for styling so child views can utilize it

[alexanderboswell]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

The FluentList and FluentItem are designed to be used together but there will need to be some coordination between them for styling. This is so that the item can be styled to match the type of list.

To achieve this, it seemed to make sense to use an environment variable so the list can set the list style and pass it to any child views that want to use it, like a ListItem view. This way implementors don't need to set a matching style on every ListItem to be the same as the style of the wrapping FluentList. We will be able to also use this value for FluentListSection, FluentListSectionHeader, and FluentListSectionFooter.

If an implementor for some reason still wanted to specify a different style on ListItem, we respect that value over the environment variable.

Updated both the background color and the spacing for the insetGrouped list style.

Binary change

Total increase: 36,016 bytes
Total decrease: 0 bytes

File	Before	After	Delta
Total	30,968,016 bytes	31,004,032 bytes	⚠️ 36,016 bytes

Full breakdown

File	Before	After	Delta
FluentList.o	49,288 bytes	70,896 bytes	⚠️ 21,608 bytes
ListItem.o	319,352 bytes	325,840 bytes	⚠️ 6,488 bytes
__.SYMDEF	4,750,168 bytes	4,756,024 bytes	⚠️ 5,856 bytes
IndeterminateProgressBar.o	231,256 bytes	232,400 bytes	⚠️ 1,144 bytes
FocusRingView.o	831,480 bytes	832,400 bytes	⚠️ 920 bytes

Verification

Visual Verification

	Before	After
header height and spacing (with a header string, with an empty string, and with no header provided)

With .grouped style specified for ListItem

With .plain style specified for ListItem

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow