Add command "reduce" (#2292)

* initial

* fold working

* tests and cleanup

* change command to reduce, with fold flag

* move complex example to tests

* add --numbered flag

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

@@ -34,6 +34,7 @@ mod parse;
 mod prepend;
 mod random;
 mod range;
+mod reduce;
 mod rename;
 mod reverse;
 mod rm;