fix(concealer): use vim.treesitter.foldexpr for stabler folding

[champignoom]

vim.treesitter.foldexpr() behaves more stably than nvim_treesitter#foldexpr()

[vhyrro]

How recent is vim.treesitter.foldexpr? Should we maybe have a fallback to the old function in case the user is running a version that doesn't have this API (vim.treesitter.foldexpr and "foldexpr" or "fallback")? Neorg should work for 0.8 and above :)