v6: introduce AnsiToken/Color styling model and align styled APIs

[dusrdev]

This PR introduces the new token-based styling model for PrettyConsole and promotes Color / Markup / AnsiToken as the preferred public surface for styled output.

Added

• Added AnsiToken as the styled ANSI abstraction for interpolated output.
• Added Color as the preferred color surface with tokens like Color.Green, Color.DefaultForeground, Color.DefaultBackground, and Color.Default.
• Added Windows ANSI capability detection through ConsoleContext.IsAnsiSupported.
• Added token-based color support to ProgressBar, Spinner, TypeWrite, and LiveConsoleRegion.RenderProgress.
• Added an implicit conversion from ConsoleColor to foreground AnsiToken for compatibility.

Breaking

• Markup now exposes AnsiToken values instead of raw string escape sequences.
• AnsiColors.Foreground(ConsoleColor) and AnsiColors.Background(ConsoleColor) now return AnsiToken instead of raw ANSI strings.

Notes

• ConsoleColor interpolation still works for compatibility.
• Low-level Write / WriteLine span APIs and Console.SetColors still use explicit ConsoleColor.
• 6.0.0 follows released 5.4.2 directly and folds in the unreleased branch work, including LiveConsoleRegion.