Minor cleanups.

Cleaned out some last remaining custome variable and ido stuff. Changes: - Added custom.el to .gitignore - Updated gc-cons-threshold comment (removed flx-ido reference) - Changed yasnippet to use yas-completing-read instead of yas-ido-prompt (works with vertico) - Removed commented-out old package list

Replace custom ibuffer code with ibuffer-project - Remove custom ibuffer major-mode grouping functions using obsolete flet - Add ibuffer-project package for automatic project-based buffer grouping - Simplify configuration from ~25 lines to ~6 lines