Correctly set fields when they are arrays.

Make error messages more user-friendly. Also remove magic constants
within functions.

Implement setFormValues for easy setting of fields.

Also migrated to pridepack because tsdx is already unmaintained.

The tests do not depend on separate files for loading templates. This is
for the layouts to be defined alongside the tests for easier updates.

The methods have been refactored and documented for ease of use of the
consumers.