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/function/newline_body.wren

@@ -0,0 +1,4 @@
+var f = new Fn {
+  // Hi.
+}
+IO.print(f.call) // expect: null

test/function/newline_in_expression_block.wren

@@ -0,0 +1,5 @@
+new Fn { IO.print("ok") // expect error
+}.call // expect error
+
+// The second error is cascaded here. If it starts failing, just remove that
+// expectation.

test/function/no_newline_before_close.wren

@@ -0,0 +1,2 @@
+new Fn {
+  IO.print("ok") }.call // expect error

test/function/syntax.wren

@@ -6,11 +6,7 @@ new Fn {
   IO.print("ok") // expect: ok
 }.call
 
-// No trailing newline.
-new Fn {
-  IO.print("ok") }.call // expect: ok
-
-// Multiple expressions.
+// Multiple statements.
 new Fn {
   IO.print("1") // expect: 1
   IO.print("2") // expect: 2