feat: add markdown plugin

lua/custom/plugins/markdown-preview.lua

@@ -0,0 +1,6 @@
+return {
+    "iamcco/markdown-preview.nvim",
+    cmd = { "MarkdownPreviewToggle", "MarkdownPreview", "MarkdownPreviewStop" },
+    ft = { "markdown" },
+    build = function() vim.fn["mkdp#util#install"]() end,
+}