fix: lspconfig.Config class not modifying inherited root_dir

[chrisgrieser]

lspconfig.Config inherits from vim.lsp.ClientConfig, where the field root_dir is typed as string. However, for the lspconfig.Config, we are also able to set a function that returns as string.

If you are trying to use lspconfig.Config to make your own config more type-safe, this therefore throws a spurious warning.