Split blocks and closures (#7075)

* Split closures and blocks

* Tests mostly working

* finish last fixes, passes all tests

* fmt

crates/nu-command/src/core_commands/def.rs

@@ -19,11 +19,7 @@ impl Command for Def {
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("def_name", SyntaxShape::String, "definition name")
             .required("params", SyntaxShape::Signature, "parameters")
-            .required(
-                "block",
-                SyntaxShape::Block(Some(vec![])),
-                "body of the definition",
-            )
+            .required("body", SyntaxShape::Closure(None), "body of the definition")
             .category(Category::Core)
     }