Avoid unwrap() if non UTF-8 in the require_literal_leading_dot flow

[Jongy]

Drop the path if UTF-8 validation fails and is required for string comparison, instead of crashing.
Add a test exhibiting the problem (fixed with this change).

Note that with require_literal_leading_dot: false, I could get results that have non utf-8 path components; with require_literal_leading_dot: true it panics without the patch, and filters those paths with it (I think that it makes more sense to filter out paths that we're unsure if should be filtered in).

An alternative is to use bytes comparison:

diff --git a/src/lib.rs b/src/lib.rs
index 8428a9a..ded971e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -943,8 +943,13 @@ fn fill_todo(
             match dirs {
                 Ok(mut children) => {
                     if options.require_literal_leading_dot {
-                        children
-                            .retain(|x| !x.file_name().unwrap().to_str().unwrap().starts_with('.'));
+                        children.retain(|x| {
+                            !x.file_name()
+                                .unwrap()
+                                .as_encoded_bytes()
+                                .first()
+                                .is_some_and(|&b| b == b'.')
+                        });
                     }
                     children.sort_by(|p1, p2| p2.file_name().cmp(&p1.file_name()));
                     todo.extend(children.into_iter().map(|x| Ok((x, idx))));

but I think that it's not a good idea to add half-done utf-8 handling. Instead, let's wait for proper support across the crate.

Btw, the rust issue rust-lang/rust#9639 is referenced around here - perhaps we should create a tracking issue in this repository and update the references?