Purpose / Goal

Some type definitions for external API, attempt to fix #69.

This PR doesn't change anything at runtime, it's only typing support for users use typescript.

It's not yet complete, but it is a start and better than nothing.

A different approach to create a separate typings file would be to convert the whole project to typescript. That would be a pretty big step and would also influence further development workflow, build process etc.

Outlook

Besides filling the gaps of this type definitions, it might be interesting to provide a second signature to parse which takes a generic type. This could then be used as the return value.
Thus, when the developer knows that the parsed XML will be in a certain structure,
It might look something like this, but I didn't try it:

export function parse(xmlData: string, options?: X2jOptionsOptional): any;
export function parse<T>(xmlData: string, options?: X2jOptionsOptional): T;

Usage would then be parse<Whatever>(myXmlData). In this case, Typescript would know that the output is of type Whatever instead of any. Without this extra definition, a similar result can be achieved with parse(myXmlData) as Whatever. Neither of those has any runtime impact, it's just at development time. I'm not sure if it is good practice, so I left it out for now.

Type

[ ]Bug Fix
[ ]Refactoring / Technology upgrade
[x]New Feature