fix(json-logic): add custom operators on headless form creation

[eshiota]

Currently all custom JSON Logic operators are applied ad-hoc when the validation functions are executed, and then removed afterwards. While this makes sense from the point of view of simulating a deterministic call to JSON Logic, other places on the form's lifecycle require these custom operators to be available.

This means that if a form is loaded with an initial value where a property needs a computed value that relies on a custom operator, the operator won't be available (as it's only added when a validation happens), thus throwing an error.

This PR simplifies the logic by adding all custom operators at once when the createHeadlessForm is called, which happens only once per page load.

[danielcardoso5]

@eshiota needs tests fixing

[eshiota]

I'm fixing the test and I kinda get why this was previously implementing trying to make the createHeadlessForm more deterministic: if this is not done, then all calls to it (either on a page with multiple forms, or over multiple calls across multiple pages) will end up sharing and accumulating operators.

For most cases this should be fine, but it could cause nasty runtime bugs where subsequent calls impact what was previously defined, or there could be cases where a form works only because of another call introduced the necessary operator.

I'll go back to the drawing board and keep the setup-and-teardown strategy in a way that works on all necessary parts.

[eshiota]

@danielcardoso5 I've opened a new PR: #238