Add regex matching support

[ChrisNeedham24]

Seems like it'll probably be faster than glob matching, and it would be good for users to have the option of using it. When doing this, glob matching should change to a -g/--glob-pattern flag and regex matching should be -r/--regex-pattern.

[ChrisNeedham24]

Acceptance Criteria:

• Add support for regex matching on file names in handle_directory(), in a similar way to how glob matching currently works.
• Change name_pattern in LffArgs to glob_pattern.
• Add a regex_pattern attribute to LffArgs.
• Use either the standard Rust regex crate or fancy_regex for this.