Move nom into nom_core workspace and re-export

[Ben-PH]

This allows for the re-export of not only nom-core, thus making this a no-change to the api, it also allows for reexport of other community extensions, such as nom-supreme

• Add nom-supreme as a (feature-gated) first-class citizen #1785
• re-export nom-supreme behind feature flag #1786

I hope to follow up on this PR by

• re-exporting nom-supreme behind a feature flag
• re-exporting nom-locate behind a feature flag
• moving some of the less "core" elements of nom into a re-exported workspace, putting them behind (default) feature flags.

The goal is to be able to integrate the nom ecosystem so that, instead of having to run

cargo add nom
cargo add nom-supreme
cargo add nom-locate

you can:

cargo add nom -F supreme locate

extra bit of fun:

to keep with the theme of this crate, we could also start adding "menu items". "extra toppings" (e.g. more verbose debug tracing). "amuse bouche" (e.g. exercise examples: have test cases and unimplemented parsers and implement them locally).