Add "trigger plural on boolean" and "trigger plural with variables on boolean" methods

[dm20]

I wrote a small implementation of the gettext concept recently and wanted to provide two new methods to the community. I was writing these in React Native, as shown below, and would be happy to incorporate the pure JS equivalents into this project for Hacktoberfest! 🎃🎃🎃

In short, the first method pboolgettext returns the plural form if a boolean is true or the singular form if the boolean is false. This is a case when no variables need to be inserted.

The second, pvboolgettext, does the same, but formats the plural message with variables.

/*
    * Returns the singular form of the message if a given boolean condition is true, otherwise the plural form.
    *
    * usage : pgettext(singularMessageId: string, pluralMessageId: string, b: boolean)
    *
    * @param singularMessageId <string> : the message ID to be used if the number of elements specified is zero
    * @param pluralMessageId <string> : the message ID to be sued if the number of elements specified is greater than zero
    * @param b <boolean> : a condition indicating if the singular message form should be displayed or not
     */
    pboolgettext(singularMessageId: string, pluralMessageId: string, b: boolean) {
        if (typeof singularMessageId === "undefined" || typeof singularMessageId === "undefined" || typeof b === "undefined") {
            throw "pboolgettext: Invalid arguments";
        }
        return b ? this.translations[singularMessageId] : this.translations[pluralMessageId];
    }

    /*
    * Returns the singular form of the message if a given boolean condition is true, otherwise the plural form formatted with variables.
    *
    * usage : pgettext(singularMessageId: string, pluralMessageId: string, b: boolean, value: any)
    * usage : pgettext(singularMessageId: string, pluralMessageId: string, b: boolean, values: [any, ... ,any])
    *
    * Note : variables can be mixed string and number.
    *
    * @param singularMessageId <string> : the message ID to be used if the number of elements specified is zero
    * @param pluralMessageId <string> : the message ID to be sued if the number of elements specified is greater than zero
    * @param b <boolean> : a condition indicating if the singular message form should be displayed or not
    * @param v <any> : the variable or variables to insert into the plural string
     */
    pvboolgettext(singularMessageId: string, pluralMessageId: string, b: boolean, ...values) {
        if (typeof singularMessageId === "undefined" || typeof singularMessageId === "undefined" || typeof b === "undefined" || typeof values === "undefined") {
            throw "pvboolgettext: Invalid arguments";
        }
        if (b) {
            return this.translations[singularMessageId];
        }
        return this.vngettext(pluralMessageId, ...values);
    }