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

crates/nu-command/tests/commands/str_/into_string.rs

@@ -45,7 +45,7 @@ fn from_boolean() {
     let actual = nu!(
         cwd: ".", pipeline(
         r#"
-        echo $true | into string
+        echo true | into string
         "#
         )
     );