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/nuon/src/to.rs

@@ -39,7 +39,7 @@ pub enum ToStyle {
 
 /// convert an actual Nushell [`Value`] to a raw string representation of the NUON data
 ///
-/// > **Note**  
+/// > **Note**
 /// > a [`Span`] can be passed to [`to_nuon`] if there is context available to the caller, e.g. when
 /// > using this function in a command implementation such as [`to nuon`](https://www.nushell.sh/commands/docs/to_nuon.html).
 ///
@@ -84,12 +84,6 @@ fn value_to_string(
             }
             Ok(format!("0x[{s}]"))
         }
-        Value::Block { .. } => Err(ShellError::UnsupportedInput {
-            msg: "blocks are currently not nuon-compatible".into(),
-            input: "value originates from here".into(),
-            msg_span: span,
-            input_span: v.span(),
-        }),
         Value::Closure { .. } => Err(ShellError::UnsupportedInput {
             msg: "closures are currently not nuon-compatible".into(),
             input: "value originates from here".into(),