Experiment: Allow both $true/true and $false/false (#4696)

* Change true/false to keywords

* oops, clippy

* Both kinds of bools

* Add in some boolean variables

* disable py virtualenv test for now

tests/shell/pipeline/commands/internal.rs

@@ -563,7 +563,7 @@ fn proper_shadow_let_aliases() {
     let actual = nu!(
         cwd: ".",
         r#"
-        let DEBUG = $false; echo $DEBUG | table; do { let DEBUG = $true; echo $DEBUG } | table; echo $DEBUG
+        let DEBUG = false; echo $DEBUG | table; do { let DEBUG = true; echo $DEBUG } | table; echo $DEBUG
         "#
     );
     assert_eq!(actual.out, "falsetruefalse");