Refactor fs.fnl to use nvim 0.8 builtins

[saccarosium]

Hi,
I've seen that in the README was stated that Conjure works with neovim 0.7 or above. I've found in fs.fnl that vim.fse.normalize is been used, witch is a nvim 0.8+ feature so I've updated the minimum version required in the README and plugin/conjure.vim. With this version bump we unlock a lot of functionality that can replace some of the old batteries that were created.

I've also took the liberty to convert the plugin/conjure.vim to lua so we can use vim.notify instead of echoerr. vim.notify can integrate with other plugins and change the displaying of notifications (see example noice.nvim)