Move from source to source-env (#6277)

* start working on source-env

* WIP

* Get most tests working, still one to go

* Fix file-relative paths; Report parser error

* Fix merge conflicts; Restore source as deprecated

* Tests: Use source-env; Remove redundant tests

* Fmt

* Respect hidden env vars

* Fix file-relative eval for source-env

* Add file-relative eval to "overlay use"

* Use FILE_PWD only in source-env and "overlay use"

* Ignore new tests for now

This will be another issue

* Throw an error if setting FILE_PWD manually

* Fix source-related test failures

* Fix nu-check to respect FILE_PWD

* Fix corrupted spans in source-env shell errors

* Fix up some references to old source

* Remove deprecation message

* Re-introduce deleted tests

Co-authored-by: kubouch <kubouch@gmail.com>

crates/nu-cli/tests/completions.rs

@@ -79,7 +79,7 @@ fn dotnu_completions() {
     let mut completer = NuCompleter::new(std::sync::Arc::new(engine), stack);
 
     // Test source completion
-    let completion_str = "source ".to_string();
+    let completion_str = "source-env ".to_string();
     let suggestions = completer.complete(&completion_str, completion_str.len());
 
     assert_eq!(1, suggestions.len());