use dynamic plugins instead of static, built-in modules

I've read some resources that say it can't be done, some that say rustc supports it but cargo doesn't, and others that say it's totally supported.

it seems available, and it would make tantivy development easier... (performance dropped 50% with it built-in, and another 50% with decent usage in lighttouch, so we did cfg feature)

I'm also worried about performance continuing to degrade, or suddenly degrading significantly, unexpectedly from anyone of these libraries due to some unexpected complexity, like with tantivy

this would also make development a lot faster with a package manager including plugins rather than depending on this project for everything

https://news.ycombinator.com/item?id=16736725
https://michael-f-bryan.github.io/rust-ffi-guide/dynamic_loading.html#lets-make-a-plugin
`rust-lang/cargo/issues/4538`