TimestampFormatter / time_diff missing config schema

Problem/Motivation

In #2921810: Allow TimestampFormatter to show as a fully cacheable time difference with JS, a time diff formatting option is provided. Part of that implementation was a "fallback configuration", labeled 'description'. No config schema was added for description, leading to situations where my CI builds that validate configuration on the site fail.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet