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/setter/associativity.wren

@@ -1,6 +1,6 @@
 class Foo {
   new(value) { _value = value }
-  toString { return _value }
+  toString { _value }
   bar = value {
     _value = value
     return value

test/setter/grouping.wren

@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { return value }
+  bar = value { value }
 }
 
 var foo = new Foo

test/setter/infix_operator.wren

@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { return value }
+  bar = value { value }
 }
 
 var foo = new Foo

test/setter/is.wren

@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { return value }
+  bar = value { value }
 }
 
 var foo = new Foo

test/setter/prefix_operator.wren

@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { return value }
+  bar = value { value }
 }
 
 var foo = new Foo

test/setter/result.wren

@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { return "result" }
+  bar = value { "result" }
 }
 
 var foo = new Foo