vfmt: fix error with `return // comment` (#6914)

vlib/v/fmt/tests/comments_expected.vv

@@ -1,6 +1,6 @@
 fn fun() int {
 	// comment zero
-	return 0
+	return 0 // another comment
 }
 
 fn mr_fun() (int, int) {
@@ -34,4 +34,6 @@ fn main() {
 	else {
 		println('else')
 	}
+	// empty return
+	return
 }

vlib/v/fmt/tests/comments_input.vv

@@ -1,5 +1,5 @@
 fn fun() int {
-	return /* comment zero */ 0
+	return /* comment zero */ 0 // another comment
 }
 
 fn mr_fun() (int, int) {
@@ -22,4 +22,5 @@ fn main() {
 	else /* after else */ {
 		println('else')
 	}
+  return // empty return
 }

vlib/v/parser/parser.v

@@ -1737,13 +1737,16 @@ fn (mut p Parser) return_stmt() ast.Return {
 	first_pos := p.tok.position()
 	p.next()
 	// no return
+	mut comments := p.eat_comments()
 	if p.tok.kind == .rcbr {
 		return ast.Return{
+			comments: comments
 			pos: first_pos
 		}
 	}
 	// return exprs
-	exprs, comments := p.expr_list()
+	exprs, comments2 := p.expr_list()
+	comments << comments2
 	end_pos := exprs.last().position()
 	return ast.Return{
 		exprs: exprs