Enhancements

• Optimize performance.
• Improve popwin.
• Use symbol-overlay instead of highlight-symbol.
• Remove back-button: using C-x C-SPC and C-u C-SPC.
• Use native rectangle-mark-mode. Remove cua-rect.
• Use treemacs instead of neotree and imenu-list.
• Enhance go-mode.
• Support emojis.