🚀 Feature: Allow Blocks to infer Addons from disk

[JoshuaKGoldberg]

Bug Report Checklist

• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

As of #88 -> #95, config files can specify Addons. Which is great! I should theoretically be able to make a create.config.js in any of my repos that use create-typescript-app and run npx create typescript-app without clobbering over changes specified as Addons.

For example, to add some rules to an eslint.config.js the way CTA does:

// eslint.config.js
// ...
rules: {
	// These on-by-default rules work well for this repo if configured
	"@typescript-eslint/prefer-nullish-coalescing": [
		"error",
		{ ignorePrimitives: true },
	],
	"@typescript-eslint/restrict-template-expressions": [
		"error",
		{ allowBoolean: true, allowNullish: true, allowNumber: true },
	],
// ...

...a create.config.js can specify those in the ESLint Block:

// create.config.js
import { createConfig } from "create";
import { presetEverything } from "create-typescript-app";

export default createConfig(presetEverything, {
	addons: [
		blockESLint({
			rules: [
				{
					comment:
						"These on-by-default rules work well for this repo if configured",
					entries: {
						"@typescript-eslint/prefer-nullish-coalescing": [
							"error",
							{ ignorePrimitives: true },
						],
						"@typescript-eslint/restrict-template-expressions": [
							"error",
							{ allowBoolean: true, allowNullish: true, allowNumber: true },
						],
					},
				},
			],
		}),
	],
});

But, manually specifying Addons in a create.config.js is clunky. It means you have those changes specified in two places at once. I don't think most end-users would be happy with a system requiring that. I certainly wouldn't be.

I think the only way to get this right would be to make Addons _automatically inferable from files on disk. As in, that create.config.js should be able to not manually specify any Addons:

import { createConfig } from "create";
import { presetEverything } from "create-typescript-app";

export default createConfig(presetEverything);

...and instead read the existing eslint.config.js to parse the addons from the text in the file:

{
	"comment": "These on-by-default rules work well for this repo if configured",
	"entries": {
		"@typescript-eslint/prefer-nullish-coalescing": [
			"error",
			{ "ignorePrimitives": true }
		],
		"@typescript-eslint/restrict-template-expressions": [
			"error",
			{ "allowBoolean": true, "allowNullish": true, "allowNumber": true }
		]
	}
}

I'm very confident this is the right direction to take the engine, but I don't yet know how to do it. I don't know if Handlebars or Mustache can theoretically support such a reverse-mapping. There might be an existing third-party project but I wouldn't be surprised if not. If they can't theoretically support this, I might have to make some kind of more direct, less-featured custom templating engine. 😬

Additional Info

Marking as Blocked until at least the Blocks Launch milestone is done. This is pretty tricky and should probably come after the templating engine is made more pluggable per #76.

💝