Support other variables than PATH in pathvar (#3791)

* Fix swapped PATH env var separators

* Support pathvar to manipulate other vars than PATH

* Add tests for pathvar and its subcommands

* Fix PATH env name for Windows

Seems like Windows uses PATH as well.

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;