- Move `help` commands from `nu-cmd-lang` back to `nu-command`
- Make `nu-cmd-lang` only a dev dependency for `nu-command`
- Change context creation helpers for `nu-cmd-extra` and
`nu-cmd-dataframe` to have a consistent api used in
`src/main.rs`:`get_engine_state()`
- `nu-command` now indepedent from `nu-cmd-extra` and `nu-cmd-dataframe`
that are now dependencies of `nu` directly. (change to internal features)
- Fix tests that previously used `nu-command::create_default_context()`
with replacement functions

Move `nu-cmd-lang` to dev-dependencies as no longer needed for the crate
function only relevant in tests.