Explore Scripting extensions

[alandefreitas]

This is a sub-issue of #1113.

#1113 already lists #881, but #881 is much more specific.

[gennaroprota]

Resolved by #1196, which shipped scripting extensions: Lua Handlebars helpers and Lua/JS corpus-mutation extensions. Follow-on work has dedicated issues: #1210 (entry-point ladder beyond transform_corpus), #1213 (ad-hoc predicate filtering in Handlebars), and #1215 (computed properties via reflection).