Implement SkuConfigUpdater for codemodding sku config

[askoufis]

As part of the upcoming eslint v9 breaking change, the need has arisen to modify consumer's sku config to enable a smoother migration. This PR implements a SkuConfigUpdater class that provides a simple interface for performing imperative updates to a sku config, abstracting away the AST parsing, modification and serialization.

Reviewer notes

AST transformation code can be hard to understand just via visual inspection. The majority of it is just narrowing types to keep the typechecker happy. I wouldn't waste much time reviewing the AST code, the test cases showcase the API much more clearly.

API Example

Below is an example of how the future eslintIgnore option will be inserted into a user's sku config:

// A static method is used here because class constructors can't be async
const updater = await SkuConfigUpdater.fromFile(pathToSkuConfig);

// Modifies the AST of the sku config stored by `updater`
updater.upsertConfig('eslintIgnore', ['**/dist', 'src/types/graphql.ts']);

// Serializes (writes) the AST to disk
await updater.commitConfig();

See the test cases for more examples.

Implementation

SkuConfigUpdater primarily abstracts over magicast, which itself is an abstraction over recast, but uses babel to parse ASTs rather than acorn, and provides a nice Proxy-based API for modifying module imports/exports.

While magicast's API is nice, it doesn't handle CJS exports at all, so I went through a bit of effort implementing logic to deal with sku configs that use module.exports. It's not as clean as I'd like, but I think future effort could be put into automatically uplifting non-TS and/or non-ESM sku configs to TS + ESM, potentially enabling sku to drop support for CJS config entirely.

Dependencies

All dependencies of magicast are already in sku's dependency tree, and magicast itself isn't that large (~120kb), so IMO there's no cause for concern.

Testing

Used this new API to test out fs-fixture, a simple library for creating disposable test fixtures on disk. I think it's pretty neat, and has enabled a relatively simple way to write full integration tests for SkuConfigUpdater solutions that involve mocking the file system.

Misc.

SkuConfigUpdater isn't a great name, definitely open to better suggestions.

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