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/let_env.rs

@@ -20,7 +20,7 @@ impl Command for LetEnv {
             .required("var_name", SyntaxShape::String, "variable name")
             .required(
                 "initial_value",
-                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::String)),
+                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::Any)),
                 "equals sign followed by value",
             )
             .category(Category::Env)
@@ -42,9 +42,6 @@ impl Command for LetEnv {
             .expect("internal error: missing keyword");
 
         let rhs = eval_expression(engine_state, stack, keyword_expr)?;
-        let rhs = rhs.as_string()?;
-
-        //println!("Adding: {:?} to {}", rhs, var_id);
 
         stack.add_env_var(env_var, rhs);
         Ok(PipelineData::new(call.head))