feat!: ESLint 8 and jsonc-eslint-parser

[JoshuaKGoldberg]

Fixes #39; fixes #43.

Moves to the external jsonc-eslint-parser because:

• Tooling: consider switching to jsonc-eslint-parser #43: it's a good ecosystem standard
• ESLint's RuleTester doesn't support this usage of processors (feat!: ESLint 8 and jsonc-eslint-parser #40 (comment))

Also removes the disparity dependency, in favor of directly telling users to run the auto-fixer. This is the strategy other plugins such as eslint-plugin-simple-import-sort take. It's simpler to implement, makes for easier-to-read error messages, and reduces the size of node_modules.

[JoshuaKGoldberg]

From the ESLint Discord: https://discord.com/channels/688543509199716507/711686958635548723/1151961528011063296

Question: I just started overhauling an old plugin that uses a processor. I'd like to get it to use RuleTester for the rules. Is that blocked on eslint/rfcs#31 (feat: RuleTester supports processor)?

Response: Short answer: yes, you're blocked. Longer answer: it's not clear that the RFC is the right solution to this problem, and because you're not using JS, you are in a sort of "voiding your warranty" situation. 🙂
It's possible that language plugins would make this easier to solve, but there's no straightforward answer right now.

Welp.