Gutenberg - Form block: restrict phone field to numeric data

We received a request in #1846407-zen to be able to restrict the phone field in the Form block to accept numeric data only.

I did mention in my reply that this may be partly due to the way country codes are expressed, e.g. `+1`, `+44` rather than `001` or `0044`, but thought the request might be worth mentioning here.

Would something like an (optional) drop-down menu to select country code, then a numeric-only field for the number work?

The issue names the Gutenberg Form block but no files or tests; start by locating that block and reviewing how its phone field currently handles input. Clarify whether country-code selection and numeric-only validation are both required, then define completion around the agreed behavior and its coverage in tests.

Written by the indexing model from the issue text.