[add] markdown preview for mermaid code

lua/plugins/markdown-preview.lua

@@ -0,0 +1,12 @@
+return {
+  "selimacerbas/markdown-preview.nvim",
+  dependencies = { "selimacerbas/live-server.nvim" },
+  config = function()
+    require("markdown_preview").setup({
+      -- all optional; sane defaults shown
+      port = 8421,
+      open_browser = true,
+      debounce_ms = 300,
+    })
+  end,
+}