feat: improve flat config errors for invalid rule options and severities

[JoshuaKGoldberg]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[x] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Adds a couple of new error messages for flat config schema parsing.

Fixes #17056

Is there anything you'd like reviewers to focus on?

• Unit tests don't seem to cover the full error messages - is this something you'd want added in?
• Please bikeshed my error messages 😄 I'm not confident in them, and mostly based them off the legacy config system's.
  • Additionally, this removes the Key: ... portion of the error message, in favor of the traditional Configuration for rule .... I see this change as more readable, perhaps?

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[nzakas]

Thanks for taking a stab at this. In general, I think this is a good change. I'd just prefer to keep the original error message on the error objects themselves so we have consistency with how errors are reported in the code. Users won't end up seeing those errors messages anyway.

And yes, we don't test verbose error messages because those just bubble up to the CLI. We want to make sure that the thrown error matches the expectation and then we let the CLI do its thing to make those errors pretty.

[nzakas]

LGTM! Just want @mdjermanovic to give this a review before merging.

[mdjermanovic]

LGTM, thanks!