fix: make types usable in CommonJS

[fasttime]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Similar to eslint/rewrite#143 and eslint/json#77, this PR ensures that the @eslint/css types can be imported from a CommonJS module when moduleResolution is one of 'node16' or 'nodenext'. This is currently not possible because of an issue with the types in @eslint/plugin-kit that has been fixed in the latest version v0.2.5.

What changes did you make? (Give an overview)

• Updated dependency @eslint/plugin-kit to version spec "^0.2.5".
• Made @eslint/core a runtime dependency of this package.
• Added @types/css-tree as a runtime dependency.
• Added tests to ensure that @eslint/css can be imported from a .cts file.

Related Issues

refs eslint/rewrite#143, refs eslint/json#77.

Is there anything you'd like reviewers to focus on?

[fasttime]

The generated dist files dist/cjs/index.d.cts and dist/esm/index.d.ts contain type imports from css-tree, for example:

export type CssNode = import("css-tree").CssNode;

css-tree does not contain built-in types, so it seems good to add a dependency on the types package. If we don't want to add a runtime dependency I think we should at least keep @types/css-tree in the devDependencies so that the types test can validate our own types against the css-tree types.