Better align TypeScript type checking in web-forms, .vue modules

As I've been reviewing #183, trying a change I want to suggest produced a very surprising type for a destructured array member. What surprised me is that I expected it to be `string | undefined`, but it is not!

As it turns out, the TypeScript configurations in `@getodk/web-forms` are much less strict than throughout the rest of the project. We should make these consistent. We should also consider sharing a great deal more between configs so it's less likely for discrepancies like this to occur.

Inspect the TypeScript configurations in @getodk/web-forms and compare them with the configurations used throughout the rest of the project. Reproduce the destructured-array type-checking example from the issue, then make the configurations consistent and verify that the unexpected type difference is resolved.

Written by the indexing model from the issue text.