Make it easier to test pattern restrictions while editing

The pattern restriction setting for a "mathematical expression" part is not easy to write. It would be nice to have an area for testing expressions against the pattern, to see if they match and what the captured terms are.

Start by locating the editor UI for pattern restrictions on mathematical expression parts. Add an area where expressions can be tested against the pattern, showing whether they match and which terms were captured; done means both outcomes are visible while editing.

Written by the indexing model from the issue text.