DatePicker set first day of week and remove manual translation files

[tanthammar]

This PR suggests the following changes to the DatePicker field:

• Remove the need for manual translation files
• Set any weekday as first day of week
• Datepicker language defaults to app()->getLocale()
• config option: first-day-of-week

Added methods:

/**
     * Default = app()->getLocale()
     * DayJS i18 via browser: https://day.js.org/docs/en/i18n/loading-into-browser
     * Available methods: https://day.js.org/docs/en/plugin/locale-data
     * Supported locales: https://github.com/iamkun/dayjs/tree/dev/src/locale
     * @param string $dayJSlocale
     * @return $this
     */
    public function locale(string $dayJSlocale): self
    {
        $this->locale = $dayJSlocale;
        return $this;
    }

    /**
     * 0 to 7 are allowed. Monday = 1, Sunday = 0 or 7
     * @param int $day
     * @return $this
     */
    public function firstDayOfWeek(int $day = 1): self
    {
        if($day < 0 || $day > 7) $day = 1;
        $this->firstDayOfWeek = $day;
        return $this;
    }

    /**
     * Set Sunday as first day of week
     * @return $this
     */
    public function weekStartSunday(): self
    {
        $this->firstDayOfWeek = 7;
        return $this;
    }

    /**
     * This method is otiose because Monday is default
     * <br>But sometimes you want it for code readability
     * @return $this
     */
    public function weekStartMonday(): self
    {
        $this->firstDayOfWeek = 1;
        return $this;
    }


// + equivalent getters...

[tanthammar]

Ouch, forgot to add class "capitalize"

[danharrin]

Thanks Tina! I've removed the unnecessary translation strings as well.