Add typings for currency fields in GlideElement

[knikolov-nuvolo]

When the field is of type Currency, the GlideElement object has additional methods:
https://docs.servicenow.com/bundle/newyork-platform-administration/page/administer/currency/concept/currency-values-scripts.html

These methods are used for i18n, so we need them.

My proposal is to add class for the Currency type, that extends GlideElement and adds the additional methods. Example:

declare class GlideCurrencyElement extends GlideElement {
  getReferenceValue(): string;
  getSessionValue(): string;
  // etc.
}

This should require a change in the script that generates the table definitions.
Additionally, all existing tables should be updated.

[knikolov-nuvolo]

As a quick temporary solution, I'll add the methods in GlideElement with comments notifying the user that they are specific for Currency fields only.

In future we should provide a better solution.