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-cli/src/completions.rs

@@ -70,9 +70,7 @@ impl NuCompleter {
     ) -> Vec<(reedline::Span, String)> {
         let mut output = vec![];
 
-        let builtins = [
-            "$nu", "$scope", "$in", "$config", "$env", "$true", "$false", "$nothing",
-        ];
+        let builtins = ["$nu", "$scope", "$in", "$config", "$env", "$nothing"];
 
         for builtin in builtins {
             if builtin.as_bytes().starts_with(prefix) {