fix(markdown-preview): unknown function mkdp#util#install (#4196)

## Description
This fixes the problem when checking for updates and trying to build and
the user encounters the error `unknown function mkdp#util#install`.

As per https://github.com/iamcco/markdown-preview.nvim/pull/691
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## Related Issue(s)
No, rather a discussion #4194
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  If this PR fixes any issues, please link to the issue here.
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## Screenshots

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## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.

lua/lazyvim/plugins/extras/lang/markdown.lua

@@ -76,6 +76,7 @@ return {
     "iamcco/markdown-preview.nvim",
     cmd = { "MarkdownPreviewToggle", "MarkdownPreview", "MarkdownPreviewStop" },
     build = function()
+      require("lazy").load({ plugins = { "markdown-preview.nvim" } })
       vim.fn["mkdp#util#install"]()
     end,
     keys = {