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.

crates/nu-cmd-lang/src/example_support.rs

@@ -288,9 +288,6 @@ impl<'a> std::fmt::Debug for DebuggableValue<'a> {
                 }
                 write!(f, "]")
             }
-            Value::Block { val, .. } => {
-                write!(f, "Block({:?})", val)
-            }
             Value::Closure { val, .. } => {
                 write!(f, "Closure({:?})", val)
             }