[utils] [new] parse: support flat config

[DMartens]

Flat ESLint no longer supports context.parserPath which is required for utils/parse.
This PR uses context.languageOptions.parse which has the loaded parser as an alternative.
Some notes:

• Backwards compatible as it only checks if context.parserPath is unset
• Currently it does check if parse or parseForESLint in context.languageOptions.parser is a function (I do not if "constructed" context objects are officially supported)
• I added a test to the rule to no-unused-modules for testing with flat eslint rule tester

[ljharb]

will this test still work with the approach we've taken to have a config shared between flat and legacy? Meaning, the legacy config object still has languageOptions, I just mark it non-enumerable.

[DMartens]

As far as I understand it is that has nothing to do with the read config file, as context.languageOptions.parserOptions is also set for legacy configs (relevant code). But if the linter runs in "flat" mode it no longer sets the context.parserPath, context.parserOptions ... .

[ljharb]

ok - it's not that languageOptions is set that IDs it as the flat config, it's that it's set and has an object parser property?

Are we sure it can't be null, only a string or an object?

[DMartens]

No as I understand it in the current eslint both properties are set: context.parserOptions and context.languageOptions.parserOptions (same with other properties of languageOptions). But in "flat" mode the top-level properties are not set.

To the type of languageOptions.parser:

• null: I created an extra test case for this case.
• string: is from @types/eslint
• object: is already an object for the current eslint

[ljharb]

Let's also add runtime checks that throw errors if the types aren't what we expect.

[DMartens]

Added runtime checks that context.languageOptions.parser either has a function parse or parseForESLint

[DMartens]

Added support for context.languageOptions.parserOptions (which is preferred over context.parserOptions).
Maybe that was the cause of the confusion but I forgot to add this because as I said context.parserOptions is not set in "flat" mode.

[ljharb]

Did some tweaks for style, but also additional ones to handle that FlatRuleTester isn't available in eslint < 8.

[azat-io]

I have a lot of errors like this: error Parse errors in imported module 'vite': parserPath or languageOptions.parser is required! (undefined:undefined) import/namespace.
I use flat config, can I fix it?

[ljharb]

@azat-io meaning, with eslint-module-utils 2.7 you don't see those, but with 2.8 you do?