Bug: Project service doesn't infer compilerOptions for files not included in a tsconfig.json

[JoshuaKGoldberg]

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Issue Description

Moving #8030 (reply in thread) into a standalone issue: users are going to want to be able to lint .eslintrc.cjs files without including them in the tsconfig.json compilerOptions.allow and include. But the project service as-is provides a "blank slate" set of default compiler options when a file isn't included in a loaded tsconfig.json.

Proposal: if a file is included in allowDefaultProjectForFiles (added in #7752) and isn't given compiler options from a loaded TSConfig, instead give it the compiler options for the nearest tsconfig.json?

Reproduction Repository Link

https://github.com/JoshuaKGoldberg/repros/tree/ts-eslint-project-service-not-included

Repro Steps

1. clone the repo
2. npm install
3. npm lint

Versions

package	version
@typescript-eslint/parser	6.18.0
@typescript-eslint/typescript-estree	6.18.0
TypeScript	5.3.3

[bradzacher]

I dunno if we should magically infer the options like that. I kinda hate black-box magic like that.

I'd be happy with an option to set the tsconfig used for extra files - so people could configure it from their eslint config. Then if they wanted to explicitly use a specific config then they could require it in place.

As an example the closest tsconfig might be one for Web files and include like the DOM libs or have a really restricted es version. An explicit config would let someone tweak things for their node env

[mrazauskas]

Some defaultTSConfig option makes sense for me. It could take a path to tsconfig.json and use compiler options found there for inferred projects. Also the default value could be './tsconfig.json'. This is some magic too, but with a possibility to opt out.

I was thinking about the motivation for './tsconfig.json' default value. Probably typescript-eslint is mostly used in TypeScript projects. These often have tsconfig.json at the root and very likely this is the default TSConfig. Files which are not included in the project would be something like eslint.config.js. I would be fine to use options from tsconfig.json on these. Of course, this is just a single option with many assumptions in it (;

[JoshuaKGoldberg]

Note for reference: #8460 (review). Once this is in, we'll want to breaking change remove the main entry from typescript-eslint.