Add a ReadFile method, to read a file inside a resource pack

[smell-of-curry]

I use this a lot to read .lang files to provide translations inside the proxy. For example:

// GetLangTranslationMap returns a map of translations for a specific language from a resource pack.
func GetLangTranslationMap(pack *resource.Pack, language string) (map[string]string, error) {
	languageBytes, err := pack.ReadFile("texts/" + language + ".lang")
	if err != nil {
		return nil, fmt.Errorf("error while reading language file: %w", err)
	}

	langMap := make(map[string]string)
	scanner := bufio.NewScanner(strings.NewReader(string(languageBytes)))