Explicit category order

[cspotcode]

Implements #96

WIP to gauge if this will be accepted before I add tests.

[arcanis]

Sorry I didn't see this sooner - I think it makes sense. There's just the extra complexity in category format caching that I don't entirely understand?

[arcanis]

What are those changes for? 🤔

[cspotcode]

I see what you mean; this cache is only being read in one place, so the computation of formattedCategory should be moved down to line 458 where it is being used.

[cspotcode]

I somewhat reverted the changes. I'm not entirely sure what formatMarkdownish is doing. Which is better for commandsByCategories keys: the original category, or the markdown-formatted category?

[cspotcode]

For both this and #99, do you have any guidance on writing tests? I skimmed the tests and I can see the --help tests. However, I expected to see some snapshots of the --help output. Rather, I see the tests often comparing the output of 2x different invocations to verify that they are identical, but not necessarily to verify that the output is formatted in any particular way. I may be misunderstanding the test philosophy, and I'm hoping that you can offer a sentence or two to point me in the right direction.