add feature change package name and message type name

[mr-tron]

It can be useful for creating simple wrappers like #292 asks. For example my wrapper:

package i18n

import (
	"embed"
	"github.com/BurntSushi/toml"
	"github.com/nicksnyder/go-i18n/v2/i18n"
	"golang.org/x/text/language"
)

//go:embed locale.*.toml
var localeFS embed.FS
var bundle *i18n.Bundle

func init() {
	bundle = i18n.NewBundle(language.English)
	bundle.RegisterUnmarshalFunc("toml", toml.Unmarshal)
	bundle.LoadMessageFileFS(localeFS, "active.en.toml")
}

type C = i18n.LocalizeConfig
type M = i18n.Message

func T(lang string, c C) string {
	s, _ := i18n.NewLocalizer(bundle, lang).Localize(&c)
	return s
}

and i can use it like

import "my/wrapper/path/i18n"

...

i18n.T(i18n.C{DefaultMessage:&i18n.M{blah blah blah}})

[mr-tron]

hello?

[nicksnyder]

This change is just to enable you to write something like i18n.M instead of i18n.Message?

I am currently hesitant to merge a change like this, it seems unnecessary, and I am wondering if there is a better way to accomplish what you want (i.e. having i18n expose a type alias), but any case a change like this would needs tests before I could consider merging it.

[mr-tron]

It allows to make proxy-package inside my project. Aliases is only one example of usage.
Ok. I will try to write tests.