fix(lsp): schedule call to vim.lsp.start for async root_dir (#31998)

When `root_dir` is a function it can (and often will) call the provided
callback function in a fast API context (e.g. in the `on_exit` handler
of `vim.system`). When the callback function is executed we should
ensure that it runs vim.lsp.start on the main event loop.

runtime/lua/vim/lsp.lua

@@ -513,7 +513,9 @@ local function lsp_enable_callback(bufnr)
         ---@param root_dir string
         config.root_dir(function(root_dir)
           config.root_dir = root_dir
-          start(config)
+          vim.schedule(function()
+            start(config)
+          end)
         end)
       else
         start(config)