dynamic prefixes should not be able match without a slash seperator

[robjtede]

Good! Now we have to find a solution for dynamic prefixes. One idea is to implement it in regex, where each dynamic prefix pattern is put inside a capture group with a pattern to detect segment boundary: (original_regex_pattern)[/$]. But this may add complexity. I'm not sure.

Originally posted by @aliemjay in #373 (comment)

test case that should not work but does:

let resource = ResourceDef::prefix(r"/id/{id:\d{3}}");
assert!(resource.is_match("/id/1234"));
assert_eq!(resource.find_match("/id/1234"), Some(7));