Support other variables than PATH in pathvar (2nd attempt) (#3828)

* Fix swapped PATH env var separators

* Support pathvar to manipulate other vars than PATH

* Add tests for pathvar and its subcommands

* Adjust pathvar tests to comply with env quirks

* Make pathvar tests work on non-Windows as well

* Compact the comments in pathvar tests

* Fix last failing test

Co-authored-by: Jakub Žádník <jakub.zadnik@tuni.fi>

crates/nu-command/tests/commands/mod.rs

@@ -37,6 +37,7 @@ mod move_;
 mod open;
 mod parse;
 mod path;
+mod pathvar;
 mod prepend;
 mod random;
 mod range;