Feature idea: Per-field validation for multiple_text

- need to decide how to report multiple errors and highlight the correct fields in the form
- do we allow customization of the validation or create a bunch of separate presets `multiple_url`, `multiple_email` and so on

Start by locating the existing multiple_text validation entry point and reviewing how form errors are currently reported. Clarify whether multiple errors need separate field highlighting and whether validation should be customizable or exposed through presets such as multiple_url and multiple_email; the work is done when those choices are specified and implemented consistently.

Written by the indexing model from the issue text.