feat(diagnostics): emulate VSCode Error Lens with virtual text and lines

Added functionality to mimic VSCode's Error Lens behavior:
        - Displays diagnostics as virtual text with severity icons during insert mode.
        - Switches to virtual lines with formatted messages in normal mode for better readability.
        - Replaced `InsertLeave` with `ModeChanged` (`i:*`) to ensure diagnostics update correctly even when exiting insert>

        This refactor improves code readability, follows single-responsibility principles, and makes it easier to maintain >
        Improves real-time feedback while coding and enhances the visual presentation of diagnostics.

init.lua

@@ -672,7 +672,8 @@ require('lazy').setup({
         end,
       })
 
-      vim.api.nvim_create_autocmd('InsertLeave', {
+      vim.api.nvim_create_autocmd('ModeChanged', {
+        pattern = 'i:*',
         callback = function()
           set_virtual_text(false)
         end,