Remove the Value::Block case (#12582)

# Description
`Value` describes the types of first-class values that users and scripts
can create, manipulate, pass around, and store. However, `Block`s are
not first-class values in the language, so this PR removes it from
`Value`. This removes some unnecessary code, and this change should be
invisible to the user except for the change to `scope modules` described
below.

# User-Facing Changes
Breaking change: the output of `scope modules` was changed so that
`env_block` is now `has_env_block` which is a boolean value instead of a
`Block`.

# After Submitting
Update the language guide possibly.

tests/scope/mod.rs

@@ -134,10 +134,10 @@ fn correct_scope_modules_fields() {
 
         let inp = &[
             "use spam.nu",
-            "scope modules | where name == spam | get 0.env_block | is-empty",
+            "scope modules | where name == spam | get 0.has_env_block",
         ];
         let actual = nu!(cwd: dirs.test(), &inp.join("; "));
-        assert_eq!(actual.out, "false");
+        assert_eq!(actual.out, "true");
 
         let inp = &[
             "use spam.nu",