Support "reverse" in active-group-style for group headers

[zambetti]

Summary

Add support for reverse in active-group-style so the active completion group header can be highlighted reversed.

Changes

• Accept reverse as a valid active-group-style token in lib/-ftb-fzf.
• Apply reverse styling in lib/ftb-switch-group using ANSI SGR: on: 7 off: 27

Implementation

The previous implementation relied on hardcoded case branches for every style combination (bold, underline, bold,underline, etc.), which was rigid and didn’t scale.

This refactor takes a token-based approach, parsing style tokens once, and mapping each to its SGR codes to dynamically build the combined sequence. It also handles none more cleanly by short-circuiting. If none is present, all other style tokens are discarded and only none is applied, preventing conflicting or redundant formatting states.

This keeps behavior consistent while making it easier to extend and maintain.

Example

zstyle ':fzf-tab:*' active-group-style reverse bold