Move from internal expect rule to eslint-plugin-expect-type

[JoshuaKGoldberg]

#843 migrated the old TSLint Expect rule to an ESLint rule (🥳!). Great!

There exists a community plugin for roughly the same area of functionality already: https://github.com/JoshuaKGoldberg/eslint-plugin-expect-type ¹. It would be great if we could use the standard plugin instead. That way bugfixes + features from DT usage can help other community members, and vice versa.

Proposal:

1. For each bug or feature that is better in this repo, we can file a corresponding issue on eslint-plugin-expect-type
2. Once those are all fixed up, move to using the external plugin

Per later comments, tracking issues in eslint-plugin-expect-type for missing features:

• 🚀 Feature: Allow specifying multiple TS versions (node_modules/typescript paths) JoshuaKGoldberg/eslint-plugin-expect-type#107
• 🚀 Feature: Normalize union ordering and readonly array syntax JoshuaKGoldberg/eslint-plugin-expect-type#108
• 🚀 Feature: Allow specifying multiple possible types with || JoshuaKGoldberg/eslint-plugin-expect-type#109

Footnotes

1. I do feel a bit weird promoting a package with my name on it. If it helps, I didn't create it - just took over maintenance when the previous maintainer didn't have time. If there's a better alternative to eslint-plugin-expect-type that'd be great too! ↩

[jakebailey]

I'm not sure how we can do that. Our plugin has to loop and run on every version of typescript. That seems like a lot of extra work for an external plugin to take on (and for us to no longer be able to quickly fix).

[JoshuaKGoldberg]

At the very least, I wonder if eslint-plugin-expect-type could export a Node API (or one be extracted out, or some similar strategy) to deduplicate the code. Then it could take in getProgram as an option... 🤔

Alternately, I wonder if it would make sense to consider versionsToTest "just another" option for the rule? As in, copy that logic roughly as-is to eslint-plugin-expect-type? DefinitelyTyped is likely not the only real-world use case for wanting to run across multiple TS versions.

[jakebailey]

The only interesting things missing from your plugin are || (for multiple possible answers), and the normalization of union ordering / readonly array syntax (https://github.com/microsoft/DefinitelyTyped-tools/blob/master/packages/eslint-plugin/src/rules/expect.ts#L492), both of which are related to running on multiple versions of TypeScript.