Add Expression Matches

For a given expression, it should be possible to record known matches/non-matches with a given engine.

The default engine should be the server's current .NET Core framework regex engine.

Let the user easily add examples from the Edit Expression page.

Start with the Edit Expression page and the server's current .NET Core framework regex engine. Trace how an expression is represented and how the page records user changes. Done means users can add known matching and non-matching examples for an expression, with the default engine used when none is specified.

Written by the indexing model from the issue text.