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-color-config/src/style_computer.rs

@@ -126,9 +126,7 @@ impl<'a> StyleComputer<'a> {
             Value::Nothing { .. } => TextStyle::with_style(Left, s),
             Value::Binary { .. } => TextStyle::with_style(Left, s),
             Value::CellPath { .. } => TextStyle::with_style(Left, s),
-            Value::Record { .. } | Value::List { .. } | Value::Block { .. } => {
-                TextStyle::with_style(Left, s)
-            }
+            Value::Record { .. } | Value::List { .. } => TextStyle::with_style(Left, s),
             Value::Closure { .. }
             | Value::Custom { .. }
             | Value::Error { .. }