Improve Hubspot types

[snelsi]

It would be nice to improve typing of Hubspot fields in src/hubspot.types.ts.
Currently, most of the fields are loosely marked as 'unknown function' or 'unknown object'.
My knowledge of Hubspot is too superficial to decipher the types of the arguments by the function name.

Official documentation:
https://legacydocs.hubspot.com/docs/methods/forms/advanced_form_options

You can view all fields in your browser by calling console.log(window.hubspot) and console.log(window.hbspt).