refactor(highlights): Reload highlights on theme change

[bassamsdata]

Hello!

This PR introduces a small quality-of-life improvement to the highlight groups. It ensures they are properly reloaded when the colorscheme changes by using the ColorScheme event.

Issue: Blink highlight groups currently become empty when switching colorschemes unless Neovim is restarted.
This change should also help address issue #2199.

This is my first PR here, so please let me know if anything needs to be adjusted or added.

Thank you!

[Copilot]

Pull request overview

This PR adds automatic reloading of Blink highlight groups when the colorscheme changes by listening to the ColorScheme event. This addresses an issue where highlight groups would become empty after switching colorschemes without restarting Neovim.

Key changes:

• Refactored highlight setup into a local setup_highlights() function
• Added a ColorScheme autocmd to reload highlights on theme changes
• Modified the public highlights.setup() to set up both initial highlights and the autocmd

Comments suppressed due to low confidence (1)

lua/blink/cmp/highlights.lua:11

• The default = true option prevents highlight groups from being updated when they already exist. When the ColorScheme autocmd triggers setup_highlights(), the highlight groups will already be defined, so they won't be updated with the new colorscheme's values. This defeats the purpose of reloading highlights on colorscheme change. Remove opts.default = true or conditionally set it only on the initial setup.

  local set_hl = function(hl_group, opts)
    opts.default = true -- Prevents overriding existing definitions
    vim.api.nvim_set_hl(0, hl_group, opts)
  end

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