Add support for commenting

[dmtrKovalenko]

Neovim v10 has a built-in commenting support based on the treesitter. But rescript files can not be commented, and failing with the following error:

Option 'commentstring' is empty.

[aspeddro]

I think this should be done in https://github.com/rescript-lang/vim-rescript

Or create a file at ~/.config/nvim/after/ftplugin/rescript.lua

vim.opt_local.commentstring = '// %s'