Treating environment variables as Values (#497)

* 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

crates/nu-command/src/env/mod.rs

@@ -1,5 +1,7 @@
+mod env_command;
 mod let_env;
 mod with_env;
 
+pub use env_command::Env;
 pub use let_env::LetEnv;
 pub use with_env::WithEnv;