feat: unify block, function, and method body parsing to support single-expression implicit returns

Refactor `finishBlock` in `wren_compiler.c` to detect single-expression bodies (no newline after `{`) and leave a value on the stack, enabling implicit returns for methods and functions. Add `finishBody` wrapper to handle constructor flag. Update all benchmark and test `.wren` files to remove explicit `return` keywords from getters and single-expression methods, relying on the new implicit return behavior. Remove old `return_null_if_brace.wren` test, add `newline_body.wren`, `newline_in_expression_block.wren`, and `no_newline_before_close.wren` tests for edge cases.

test/super/closure.wren

@@ -1,13 +1,10 @@
 class Base {
-  toString { return "Base" }
+  toString { "Base" }
 }
 
 class Derived is Base {
-  getClosure {
-    return new Fn { super.toString }
-  }
-
-  toString { return "Derived" }
+  getClosure { new Fn { super.toString } }
+  toString { "Derived" }
 }
 
 var closure = (new Derived).getClosure