Make PillButtonBarItem title mutable and set its font earlier

[lcapkovic]

Platforms Impacted

• iOS
• macOS

Description of changes

Despite the naming, PillButtons and their corresponding PillButtonBarItems get used outside of the PillButtonBar.
We have a specific use where we need the title to be mutable (persistent pill button that represents a current tab switcher selection). This PR adds title mutability to the PillButtonBarItem.

This PR also includes a slight adjustment to how iOS 15+ attribute title creation is done. We now set the font as soon as possible, and only leave the color changing to the updateAppearance function. This resolves a bug where intrinsic size was incorrect before updateAppearance runs when dynamic size was enabled.

Verification

Made a new demo VC that is tailored to Pill buttons outside of the pill button bar.

Before	After

(no visual change to pill button bar intended)

Tested iOS 14 as well to verify the old non-configuration paths still work.

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)