json: allow decode/encode of alias to primitive type (#18003)

vlib/json/json_encode_primite_test.v

@@ -0,0 +1,40 @@
+import json
+
+struct Test {
+	field MySumType
+}
+
+type MyInt = int
+type MyString = string
+type MySumType = MyString | int | string
+
+fn test_alias_to_primitive() {
+	mut test := Test{
+		field: MyString('foo')
+	}
+	mut encoded := json.encode(test)
+	assert dump(encoded) == '{"field":"foo"}'
+	assert json.decode(Test, '{"field":	"foo"}')!.field == MySumType('foo')
+
+	test = Test{
+		field: 'foo'
+	}
+	encoded = json.encode(test)
+	assert dump(encoded) == '{"field":"foo"}'
+	assert json.decode(Test, '{"field":"foo"}')! == test
+
+	test = Test{
+		field: 1
+	}
+	encoded = json.encode(test)
+	assert dump(encoded) == '{"field":1}'
+	assert json.decode(Test, '{"field":1}')! == test
+
+	mut test2 := MyString('foo')
+	encoded = json.encode(test2)
+	assert dump(encoded) == '"foo"'
+
+	mut test3 := MyInt(1000)
+	encoded = json.encode(test3)
+	assert dump(encoded) == '1000'
+}