use plugin nvim-ufo for better folding

The settings are copied from https://github.com/kevinhwang91/nvim-ufo/issues/4#issuecomment-1514537245,
with some modification.

lua/config/nvim_ufo.lua

@@ -0,0 +1,48 @@
+local handler = function(virtText, lnum, endLnum, width, truncate)
+  local newVirtText = {}
+  local totalLines = vim.api.nvim_buf_line_count(0)
+  local foldedLines = endLnum - lnum
+  local suffix = (" 󰁂  %d"):format(foldedLines)
+  local sufWidth = vim.fn.strdisplaywidth(suffix)
+  local targetWidth = width - sufWidth
+  local curWidth = 0
+
+  for _, chunk in ipairs(virtText) do
+    local chunkText = chunk[1]
+    local chunkWidth = vim.fn.strdisplaywidth(chunkText)
+    if targetWidth > curWidth + chunkWidth then
+      table.insert(newVirtText, chunk)
+    else
+      chunkText = truncate(chunkText, targetWidth - curWidth)
+      local hlGroup = chunk[2]
+      table.insert(newVirtText, { chunkText, hlGroup })
+      chunkWidth = vim.fn.strdisplaywidth(chunkText)
+      -- str width returned from truncate() may less than 2nd argument, need padding
+      if curWidth + chunkWidth < targetWidth then
+        suffix = suffix .. (" "):rep(targetWidth - curWidth - chunkWidth)
+      end
+      break
+    end
+    curWidth = curWidth + chunkWidth
+  end
+  local rAlignAppndx = math.max(math.min(vim.opt.textwidth["_value"], width - 1) - curWidth - sufWidth, 0)
+  suffix = (" "):rep(rAlignAppndx) .. suffix
+  table.insert(newVirtText, { suffix, "MoreMsg" })
+  return newVirtText
+end
+
+local opts = {}
+opts["fold_virt_text_handler"] = handler
+
+require("ufo").setup(opts)
+
+vim.keymap.set("n", "zR", require("ufo").openAllFolds)
+vim.keymap.set("n", "zM", require("ufo").closeAllFolds)
+vim.keymap.set("n", "zr", require("ufo").openFoldsExceptKinds)
+vim.keymap.set("n", "<leader>K", function()
+  local winid = require("ufo").peekFoldedLinesUnderCursor()
+  if not winid then
+    -- vim.lsp.buf.hover()
+    vim.cmd([[ Lspsaga hover_doc ]])
+  end
+end)