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-protocol/src/shell_error.rs

@@ -301,6 +301,21 @@ Either make sure {0} is a string, or add a 'to_string' entry for it in ENV_CONVE
     )]
     EnvVarNotAString(String, #[label("value not representable as a string")] Span),
 
+    /// This environment variable cannot be set manually.
+    ///
+    /// ## Resolution
+    ///
+    /// This environment variable is set automatically by Nushell and cannot not be set manually.
+    #[error("{0} cannot be set manually.")]
+    #[diagnostic(
+        code(nu::shell::automatic_env_var_set_manually),
+        url(docsrs),
+        help(
+            r#"The environment variable '{0}' is set automatically by Nushell and cannot not be set manually."#
+        )
+    )]
+    AutomaticEnvVarSetManually(String, #[label("cannot set '{0}' manually")] Span),
+
     /// Division by zero is not a thing.
     ///
     /// ## Resolution