Make UCI commands show up in the help menu

[dannyhammer]

This is a cumbersome task, as it likely requires modifying the uci-parser crate.

Ideally, we'd like UCI commands to appear in the help menu. The reason this isn't done currently is because we're using clap to automatically parse all engine commands, but uci-parser uses nom to manually parse them. As far as I have found in my experiments, clap can't appropriately parse UCI commands because of all the optional arguments. It may be possible, but I haven't found a way.

Anyway, it would be nice to modify... something to allow this to be possible. I don't really know where to begin. I've already tried modifying uci-parser to parse with clap, and that didn't go well. Maybe adding the UCI commands onto EngineCommand directly, and having a value_parser function for their args?