chore: move types out of config.rs into config_types.rs (#1054)

`config.rs` is already quite long without these definitions. Since they
have no real dependencies of their own, let's move them to their own
file so `config.rs` can focus on the business logic of loading a config.

codex-rs/tui/src/markdown.rs

@@ -1,5 +1,5 @@
 use codex_core::config::Config;
-use codex_core::config::UriBasedFileOpener;
+use codex_core::config_types::UriBasedFileOpener;
 use ratatui::text::Line;
 use ratatui::text::Span;
 use std::borrow::Cow;