chore: replace regex with regex-lite, where appropriate (#1200)

As explained on https://crates.io/crates/regex-lite, `regex-lite` is a
lighter alternative to `regex` and seems to be sufficient for our
purposes.

codex-rs/tui/src/citation_regex.rs

@@ -1,6 +1,6 @@
 #![allow(clippy::expect_used)]
 
-use regex::Regex;
+use regex_lite::Regex;
 
 // This is defined in its own file so we can limit the scope of
 // `allow(clippy::expect_used)` because we cannot scope it to the `lazy_static!`

codex-rs/tui/src/markdown.rs

@@ -71,7 +71,7 @@ fn rewrite_file_citations<'a>(
         None => return Cow::Borrowed(src),
     };
 
-    CITATION_REGEX.replace_all(src, |caps: &regex::Captures<'_>| {
+    CITATION_REGEX.replace_all(src, |caps: &regex_lite::Captures<'_>| {
         let file = &caps[1];
         let start_line = &caps[2];