not delete buf when hide scrollbar cause it seems not necessary

[tim3nd]

Closes #580

There were errors for command-line window cause it's not allowed to delete buffer in command-line window.

At first I want to check if we are in command-line window by checking getcmdwintype(), but seems there is no need to always trying to delete buffer for scrollbar thumb and gutter. Cause blink already can reuse existing buffer for scrollbar thumb and gutter.

see below scrollbar_win:_make_win

function scrollbar_win:_make_win(geometry, hl_group)
  if self.buf == nil or not vim.api.nvim_buf_is_valid(self.buf) then self.buf = vim.api.nvim_create_buf(false, true) end

  local win_config = vim.tbl_deep_extend('force', geometry, {
    style = 'minimal',
    focusable = false,
    noautocmd = true,
  })
  local win = vim.api.nvim_open_win(self.buf, false, win_config)
  vim.api.nvim_set_option_value('winhighlight', 'Normal:' .. hl_group .. ',EndOfBuffer:' .. hl_group, { win = win })
  return win
end

If there are something I missed or got wrong, feel free to modify or give advice :)

[Saghen]

Thanks!