Migration: regex tests which write matches to @extra

[rowanseymour]

In old world, RegexTest has access to the run and adds the matches to @extra on the run. In the new world, tests are expressions which know nothing of runs.

Either:

• we introduce a mechanism for tests to return a dict that can be automatically added to @extra by the router
• we introduce an action for users to explicitly add things to extra, and allow them to add the result of an expression which returns the matching groups. We could then migrate rulesets with regex tests to nodes that include this action before the switch router
• we introduce an expression to the old world which allows users to regex parse input and see if we can change existing flows to using this in expression type rulesets instead of relying on @extra

I think I'm leaning toward the latter option because implicitly changing @extra isn't something I particularly want to support in the new world.

[rowanseymour]

We've now added support for getting match groups from the has_pattern test and regex_group expression function... but still no clear way to automatically migrate usage of @extra.1 etc in legacy flows.

[rowanseymour]

We now make regex group matches accessible via .extra on the result, and migrate @extra.1 in legacy flows to @legacy_extra.1