Improvements overall to Nu. Also among the changes here, we can also be more confident towards incorporating `3041`. End to end tests for checking envs properly exported to externals is not added here (since it's in the other PR)
A few things added in this PR (probably forgetting some too)
* no writes happen to history during test runs.
* environment syncing end to end coverage added.
* clean up / refactorings few areas.
* testing API for finer control (can write tests passing more than one pipeline)
* can pass environment variables in tests that nu will inherit when running.
* No longer needed.
* no longer under a module. No need to use super.
* Move run_script to engine
* Add which dep and feature to engine
* Change unwrap to expect
* Add wasm specification
* Remove which from default, add specification correctly
* Add nu-platform-specifics
* Move is_external_cmd to platform_specifics
* Add is_external_cmd to host and use it instead of nu_platform directly
* Clean up if else logic in is_external_cmd
* Bump nu-platform-specifics version
* Pass context to print_err
* Commit cargo.lock
* Move print functions to own module inside nu-engine
* Hypocratic change to run windows-nightly again
* Add import for Ordering
* Move printing of error to host
* Move platform specific which functionality to basic host
* Allow no use of cmd_name
* Fix windows compile issue
