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-command/src/formats/to/json.rs

@@ -124,7 +124,7 @@ pub fn value_to_json_value(v: &Value) -> Result<nu_json::Value, ShellError> {
 
         Value::List { vals, .. } => nu_json::Value::Array(json_list(vals)?),
         Value::Error { error, .. } => return Err(*error.clone()),
-        Value::Closure { .. } | Value::Block { .. } | Value::Range { .. } => nu_json::Value::Null,
+        Value::Closure { .. } | Value::Range { .. } => nu_json::Value::Null,
         Value::Binary { val, .. } => {
             nu_json::Value::Array(val.iter().map(|x| nu_json::Value::U64(*x as u64)).collect())
         }

crates/nu-command/src/formats/to/text.rs

@@ -134,7 +134,6 @@ fn local_into_string(value: Value, separator: &str, config: &Config) -> String {
             Ok(val) => local_into_string(val, separator, config),
             Err(error) => format!("{error:?}"),
         },
-        Value::Block { val, .. } => format!("<Block {val}>"),
         Value::Closure { val, .. } => format!("<Closure {}>", val.block_id),
         Value::Nothing { .. } => String::new(),
         Value::Error { error, .. } => format!("{error:?}"),

crates/nu-command/src/formats/to/toml.rs

@@ -67,11 +67,6 @@ fn helper(engine_state: &EngineState, v: &Value) -> Result<toml::Value, ShellErr
             helper(engine_state, &collected)?
         }
         Value::List { vals, .. } => toml::Value::Array(toml_list(engine_state, vals)?),
-        Value::Block { .. } => {
-            let code = engine_state.get_span_contents(span);
-            let code = String::from_utf8_lossy(code).to_string();
-            toml::Value::String(code)
-        }
         Value::Closure { .. } => {
             let code = engine_state.get_span_contents(span);
             let code = String::from_utf8_lossy(code).to_string();

crates/nu-command/src/formats/to/yaml.rs

@@ -75,7 +75,6 @@ pub fn value_to_yaml_value(v: &Value) -> Result<serde_yaml::Value, ShellError> {
 
             serde_yaml::Value::Sequence(out)
         }
-        Value::Block { .. } => serde_yaml::Value::Null,
         Value::Closure { .. } => serde_yaml::Value::Null,
         Value::Nothing { .. } => serde_yaml::Value::Null,
         Value::Error { error, .. } => return Err(*error.clone()),