feat(lsp): add a start function (#18631)

A alternative/subset of https://github.com/neovim/neovim/pull/18506  that should be forward compatible with a potential project system.

Configuration of LSP clients (without lspconfig) now looks like this:

    vim.lsp.start({
       name = 'my-server-name',
       cmd = {'name-of-language-server-executable'},
       root_dir = vim.fs.dirname(vim.fs.find({'setup.py', 'pyproject.toml'}, { upward = true })[1]),
    })

runtime/lua/vim/fs.lua

@@ -37,6 +37,9 @@ end
 ---@param file (string) File or directory
 ---@return (string) Parent directory of {file}
 function M.dirname(file)
+  if file == nil then
+    return nil
+  end
   return vim.fn.fnamemodify(file, ':h')
 end