fmt: add a space after + operator/method overload (#7453)

vlib/v/ast/str.v

@@ -50,6 +50,9 @@ pub fn (node &FnDecl) stringify(t &table.Table, cur_mod string) string {
 		name = 'JS.$name'
 	}
 	f.write('fn $receiver$name')
+	if name in ['+', '-', '*', '/', '%'] {
+		f.write(' ')
+	}
 	if node.is_generic {
 		f.write('<T>')
 	}

vlib/v/fmt/tests/operator_overload_keep.v

@@ -0,0 +1,11 @@
+struct Foo {
+	x int
+}
+
+fn (a Foo) + (b Foo) Foo {
+	return Foo{a.x + b.x}
+}
+
+fn (a Foo) % (b Foo) Foo {
+	return Foo{a.x % b.x}
+}