FR: List.filter()

[msuozzo]

Is your feature request related to a problem? Please describe.

I wanted to implement "labels" in a git commit message with lines of the form "label: value". Using the current Template functionality, adding a line is easy (concat(), String.join()) but removing is trickier. An LLM generated description.lines().filter(...) which looked correct before I tried it and discovered filter() was hallucinated.

Describe the solution you'd like
List.filter() in a similar form to List.map()

Describe alternatives you've considered
@bryceberger put together a workaround using List.map() which uses the empty string for excluded lines and moves the "gather" operation (join in this case) into the map expression.

description.lines().map(|line| if(<predicate>, line ++ "\n")).join("")

Still, there are clear limitations with this approach and I wouldn't expect it to be maintainable extending beyond a few successive calls. So while it does the trick, I think there's an opportunity to add a more ergonomic alternative.

Additional context
Discord discourse: https://discord.com/channels/968932220549103686/1326247959213506682