Improve help text for run, test, and dev commands

[giacomocavalieri]

This PR improves the help text for the run, dev, and test command by saying which function they're going to run in the help text. This is something people usually ignore, especially for gleam test, thinking it's doing more than it actually does. So maybe this could help clear some confusion (given someone will read the help text 😆)

[lpil]

I like the idea, but I'm not a huge fan of the syntax used here, and the line slightly long.

What does gleam help look like with this?

[giacomocavalieri]

gleam help hasn't changed:

While gleam test/dev/main --help looks like this:

[lpil]

Maybe we should specify the module name instead of the file, since it doesn't have to be at that path.

[giacomocavalieri]

I had no idea this was the case, so gleam dev runs the main function from the<PROJECT_NAME>_dev module regardless of where the module is? Does that work even if the module is under src/ or test/?

[GearsDatapacks]

That's correct. No distinction is made there

[giacomocavalieri]

I see, so should we only suggest the module name? @lpil how would you prefer that information be presented, is <PACKAGE_NAME>_dev ok?