Wrong types for `parts` on `MF.getInfo`

In TypeScript: `new MF(sourceMf).getInfo().parts` results in an error because `parts` doesn't always exist (and is not in the types).

Suggested solution: always add a `parts` array, even if there's only one part.

Start at the MF.getInfo entry point and inspect both its implementation and the TypeScript type for the returned parts value. Confirm the behavior for formulas with one and multiple parts, then update the implementation and types so parts is always an array and verify the reported TypeScript error is resolved.

Written by the indexing model from the issue text.