Case-split to handle `\cases` expression and function definitions

So far the plugin can split on `case` and `\case` expressions.

It would be nice if it could split on `\cases` expressions and function definitions as well.

Clearly, the number of alternatives to add is the product of the number of alternatives the arguments, so a split on a function definition of type `Bool -> Either () Bool -> a` would necessitate 6 alternatives. Maybe having a limit on the total number of alternatives to generate is not a totally bad idea. I don't know.

Originally considered during [my GSoC 2026 project](https://gist.github.com/Aster89/69a57c364ec2d0cc89f474cc3d194371#future-work--stretch-goals).

The issue names no files, tests, or entry point. Start by locating the plugin code and tests for splitting on `case` and `\case`, then trace how alternatives are generated for the requested `\cases` and function-definition forms. Done means both forms split correctly, with the alternative-count behavior and any generation limit addressed.

Written by the indexing model from the issue text.