* get_columns is working in the columns command
* the new location of the get_columns method is nu-protocol/src/column.rs
* reference the new location of the get_columns method
* move get_columns to nu-engine
* Proof of concept treating env vars as Values
* Refactor env var collection and method name
* Remove unnecessary pub
* Move env translations into a new file
* Fix LS_COLORS to support any Value
* Fix spans during env var translation
* Add span to env var in cd
* Improve error diagnostics
* Fix non-string env vars failing string conversion
* Make PROMPT_COMMAND a Block instead of String
* Record host env vars to a fake file
This will give spans to env vars that would otherwise be without one.
Makes errors less confusing.
* Add 'env' command to list env vars
It will list also their values translated to strings
* Sort env command by name; Add env var type
* Remove obsolete test
