wip: Add type declarations to Modeling.js

[miyuesc]

1. Add type declarations to Modeling.js
2. The options parameter in the saveXML method is optional

[nikku]

@miyuesc We're currently in the process of generating type definitions for bpmn-js, cf. #1866. Does your PR work on top of these changes?

[nikku]

Note that with #1866 our approach changed to automatic type generation from JavaScript files. So rather than creating .d.ts files you'd fix the type definitions in the JS sources.

[miyuesc]

I'm sorry, I didn't notice it was used this way before.