fix: changing FormatHelpers.setSwiftFileProperties type function to return object

[vsjolander]

Docs specify that return type should be Object but it's string in FormatHelpers.d.ts file.

Description of changes:
changing FormatHelpers.setSwiftFileProperties type function to return object

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jorenbroekema]

Sidenote: in v4 branch this is fixed already

[vsjolander]

@jorenbroekema couldn't find this type in the v4 branch but the helpers seems to still be there. Did I overlook something maybe?

Added your changes if you want add them. Thanks for your time.

[jorenbroekema]

In v4 the type interfaces aren't separated from the JS code itself anymore, typescript support is now first-class.

If you really need the interface separated you could do this:

import { setSwiftFileProperties } from 'style-dictionary/utils';

const foo = setSwiftFileProperties(...);
//    ^^^   foo is typed implicitly because the return type of setSwiftFileProperties is known, no more need to typecast it

// if you really need the return type itself:
declare type fooType = typeof foo;

More info: https://v4.styledictionary.com/version-4/migration/#types