feat: support parse ENV var on struct default tag

[inhere]

Examples:

type MyConf struct {
	Env string `default:"${APP_ENV | dev}"`
	Debug bool `default:"${APP_DEBUG | false}"`
}

cfg := NewWithOptions("test", ParseEnv, ParseDefault)

mc := &MyConf{}
_ = cfg.Decode(mc)

Output:

&config.MyConf {
  Env: string("dev"), #len=3
  Debug: bool(false),
},