[6.x] Pass field prefix to custom conditions

[duncanmcclean]

This pull request fixes an issue where custom field conditions had no way of knowing which prefixed instance of an imported fieldset they were being evaluated for.

When a fieldset containing a custom condition is imported into a blueprint multiple times with different prefixes, the condition function receives the same values payload for every instance, making it impossible to scope the condition to the fields of the relevant fieldset instance.

This PR fixes it by passing the field's prefix into the custom condition payload, allowing conditions to be scoped dynamically:

Statamic.$conditions.add('shouldShow', ({ prefix, values }) => {
    return values[`${prefix ?? ''}my_field`] === 'something';
});

Fixes #8439