Fallback root directories for foam_ls

[FoamScience]

Although not common, the associated LSP (foam_ls) can handle single files just fine.
And Neovim's LSP client errors out if a nil root folder is returned; so we look for the git root, or just use the file's folder.

I only made changes to lsp/foam_ls.lua. Let me know if changes are also required for the one in lua/lspconfig/configs.

[justinmk]

or just use the file's folder.

Note that will result in a new client for each file.

[FoamScience]

or just use the file's folder.

Note that will result in a new client for each file.

Thanks for the quick merge! I didn't think about that! but I think it's alright, as for most users the root dir will be resolved correctly; dangling single files are not useful in this language.