Add environment variable support for modules (#331)

* Add 'expor env' dummy command

* (WIP) Abstract away module exportables as Overlay

* Switch to Overlays for use/hide

Works for decls only right now.

* Fix passing import patterns of hide to eval

* Simplify use/hide of decls

* Add ImportPattern as Expr; Add use env eval

Still no parsing of "export env" so I can't test it yet.

* Refactor export parsing; Add InternalError

* Add env var export and activation; Misc changes

Now it is possible to `use` env var that was exported from a module.

This commit also adds some new errors and other small changes.

* Add env var hiding

* Fix eval not recognizing hidden decls

Without this change, calling `hide foo`, the evaluator does not know
whether a custom command named "foo" was hidden during parsing,
therefore, it is not possible to reliably throw an error about the "foo"
name not found.

* Add use/hide/export env var tests; Cleanup; Notes

* Ignore hide env related tests for now

* Fix main branch merge mess

* Fixed multi-word export def

* Fix hiding tests on Windows

* Remove env var hiding for now

crates/nu-command/src/default_context.rs

@@ -39,7 +39,9 @@ pub fn create_default_context() -> EngineState {
             Do,
             Each,
             Echo,
+            ExportCommand,
             ExportDef,
+            ExportEnv,
             External,
             First,
             For,