Ensure @config is injected in common ancestor sheet

[RobinMalfait]

This PR fixes an issue where an @config was injected in a strange location if you have multiple CSS files with Tailwind directives.

Let's say you have this setup:

/* ./src/index.css */
@import "./tailwind-setup.css";

/* ./src/tailwind-setup.css */
@import "./base.css";
@import "./components.css";
@import "./utilities.css";

/* ./src/base.css */
@tailwind base;

/* ./src/components.css */
@tailwind components;

/* ./src/utilities.css */
@tailwind utilities;

In this case, base.css, components.css, and utilities.css are all considered Tailwind roots because they contain Tailwind directives or imports.

Since there are multiple roots, the nearest common ancestor should become the tailwind root (where @config is injected). In this case, the nearest common ancestor is tailwind-setup.css (not index.css because that's further away).

Before this change, we find the common ancestor between base.css and components.css which would be index.css instead of tailwind-setup.css.

In a next iteration, we compare index.css with utilities.css and find that there is no common ancestor (because the index.css file has no parents). This resulted in the @config being injected in index.css and in utilities.css.

Continuing with the rest of the migrations, we migrate the index.css's @config away, but we didn't migrate the @config from utilities.css.

With this PR, we don't even have the @config in the utilities.css file anymore.

Test plan

1. Added an integration test with a non-migrateable config file to ensure that the @config is injected in the correct file.
2. Added an integration test with a migrateable config file to ensure that the CSS config is injected in the correct file. h/t @philipp-spiess
3. Ran the upgrade on the https://commit.tailwindui.com project and ensured that
4. The @config does not exist in the utilities.css file (this was the first bug we solved)
5. The @config is replaced in the tailwind.css file correctly.

[philipp-spiess]

Suggested change

	plugins: [
	() => {
	// custom stuff which is too complicated to migrate to CSS
	},
	],
	theme: {
	extend: {
	colors: {
	'my-red': 'red',
	},
	},
	},

When we update the config to something that we can convert to, we noticed that the @theme now ends up in src/index.css and that the @config is still inside src/tailwind-setup.css.

[RobinMalfait]

Added a failing test for this: a8a66a4

Then fixed it: dc5cd8d

Bonus points: basically deleted most of the code in the migrate-config file