Transducer support for regex-automata

[tisonkun]

https://github.com/BurntSushi/fst/blob/a0936e9b25a888a0d5b9f94b91997216253e7088/fst-regex/Cargo.toml#L6

Coming from https://github.com/rust-lang/regex/issues/1087, I wonder if the opt-in glue of regex-automata and fst is a replace of "fst-regex" here. That said, a separate crate perhaps still fst-regex or fst-regex-automata.

The implementation can be similar to https://github.com/BurntSushi/aho-corasick/blob/56256dca1bcd2365fd1dc987c1c06195429a2e2c/src/transducer.rs while we gradually expose more options to configure.

For the first milestone, make it configurable Anchored/Unanchored. I'm not quite sure if other options are essential.

Note that this is not an urge, but leave my thoughts here. It's OK to be pending until maintainer's spare time and motivation.