enhanced native completion to expand snippets

snippet-expansion.lua

-- based on u/YungDaVinci work:
-- https://www.reddit.com/r/neovim/comments/ydlgzi/expand_lsp_snippets_with_luasnip_while_using/

vim.api.nvim_create_augroup('user-snippet-expand', {})
vim.api.nvim_create_autocmd('CompleteDone', {
  group = 'user-snippet-expand',
  desc = 'Expand LSP snippet',
  pattern = '*',
  callback = function(opts)
    local comp = vim.v.completed_item
    local item = vim.tbl_get(comp, 'user_data', 'nvim', 'lsp', 'completion_item')

    -- check that we were given a snippet
    if (
      not item
      or not item.insertTextFormat
      or item.insertTextFormat == 1
    ) then
      return
    end

    -- remove the inserted text
    local cursor = vim.api.nvim_win_get_cursor(0)
    local line = vim.api.nvim_get_current_line()
    local lnum = cursor[1] - 1
    local start_char = cursor[2] - #comp.word
    vim.api.nvim_buf_set_text(opts.buf, lnum, start_char, lnum, #line, {''})

    -- insert snippet
    local snip_text = vim.tbl_get(item, 'textEdit', 'newText') or item.insertText
    assert(snip_text, "Language server indicated it had a snippet, but no snippet text could be found!")
    require('luasnip').lsp_expand(snip_text)
  end
})