Lint the validity of help categories

[Niloth-p]

What does this PR do, and why?

Check for typos in category names. Previously, these typos would go unnoticed, with the key binding going missing from the help menu.

Outstanding aspect(s)

• Feedback welcome on the formatting and phrasing of the error message printed
• This could be put on hold for the moment and be merged along with the major changes we will be doing to lint-hotkeys after the addition of contexts. But, that might take a while.

External discussion & connections

• [ ] Discussed in #zulip-terminal in topic
• [ ] Fully fixes #
• [ ] Partially fixes issue #
• [ ] Builds upon previous unmerged work in PR #
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [ ] Merge will enable work on #

How did you test this?

• [x] Manually - Behavioral changes
• [ ] Manually - Visual changes
• [ ] Adapting existing automated tests
• [ ] Adding automated tests for new behavior (or missing tests)
• [ ] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [x] It is a minimal coherent idea
• [x] It has a commit summary following the documented style (title & body)
• [x] It has a commit summary describing the motivation and reasoning for the change
• [x] It individually passes linting and tests
• [ ] It contains test additions for any new behavior
• [ ] It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes

[Niloth-p]

Thank you for the review, @neiljp. Ah, I did not think about these details in the same manner. Thank you for your perspective.

I've actually absorbed these 2 commits into #1484's refactoring.

We could finish these here, and I'll have to rebase that branch to make the flow still make sense (I would prefer to avoid that if possible, as it could lead to making silly errors and code degradation in the worst case (like it happened once before with the hotkeys.md)). Or we could make these changes there in #1484 to keep the sequencing, if we're not in a hurry to get this merged? (That should be quite straight forward).

See

• error flag commit
• typo check commit