fix(statuscolumn): fix folds_open = true (#3756)

After some recent changes in https://github.com/neovim/neovim/pull/28709
I was getting an error when the user had `folds_open = true` in his
personal configuration whenever he pressed `o/O`. This is an attempt to
fix that.

## What is this PR for?

I was getting an error with `folds_open = true` in the user
configuration whenever pressing `o/O` or pressing `Enter` for a new
line.
The error was `Error executing lua:
...e/jrn23/projects/plugins/LazyVim/lua/lazyvim/util/ui.lua:127: attempt
to index a number value`.
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maintainers
  why we should accept this pull request. -->

## Does this PR fix an existing issue?
No
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  If this PR fixes any issues, please link to the issue here.
  Fixes #<issue_number>
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## Checklist

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

lua/lazyvim/util/ui.lua

@@ -124,7 +124,7 @@ function M.statuscolumn()
       elseif
         show_open_folds
         and not LazyVim.ui.skip_foldexpr[buf]
-        and vim.treesitter.foldexpr(vim.v.lnum):sub(1, 1) == ">"
+        and tostring(vim.treesitter.foldexpr(vim.v.lnum)):sub(1, 1) == ">"
       then -- fold start
         fold = { text = vim.opt.fillchars:get().foldopen or "", texthl = githl }
       end