Fix prefix-suffix conflict due to insertion order of trailing slashes

[Lencerf]

This PR tries to solve the following failure,

#[test]
fn matchit_test1() {
    let mut router = Router::new();
    router.insert("/f{a}o", "picture").unwrap();
    router.insert("/f{a}o/{*path}", "regex").unwrap();
}

#[test]
fn matchit_test2() {
    let mut router = Router::new();
    router.insert("/f{a}o/{*path}", "regex").unwrap();
    router.insert("/f{a}o", "picture").unwrap();
}

matchit_test1 can pass but matchit_test2 fails with a conflict.

When inserting route parameters with static suffixes, matchit checked for conflicts between newly inserted suffixes and existing ones. It had an explicit exception to allow a new suffix that only differed by an extra trailing slash (e.g., o/ vs existing o).

However, this check only evaluated one direction: if the new suffix was longer than the existing suffix. If the routes were inserted in the reverse order (e.g., o/ inserted first, then o inserted second), the router incorrectly flagged them as a prefix-suffix conflict.

This commit makes the trailing slash check bidirectional by adding an else branch to verify if the existing suffix is the one with the extra trailing slash, resolving the insertion order dependency.

This PR is created with Gemini with the prompt of analyzing and fixing test case matchit_test2.