feat: Add type definitions

[nzakas]

This pull request introduces several changes to add TypeScript support and configuration for type checking in the project. The most important changes include adding a new job to the GitHub Actions workflow, updating package.json to include TypeScript dependencies and scripts, and creating TypeScript configuration and test files.

This uses https://gist.github.com/scripthunter7/3274e68ed223102246c098dcf1a772f0 as the basis for the types. It's not complete but covers most of the API.

Needs #1 before merging.

TypeScript Integration:

• .github/workflows/build.yml: Added a new job test_types to run TypeScript checks during the CI process.
• package.json: Added types field to specify the TypeScript declaration file and updated the exports section to include the types entry. Also added a new script test:types to run TypeScript checks and included typescript as a development dependency. [1] [2] [3]

TypeScript Configuration and Tests:

• tests/types/tsconfig.json: Created a TypeScript configuration file for the type tests with strict settings and noEmit option.
• tests/types/types.test.ts: Added a comprehensive TypeScript test file to validate type definitions and ensure proper type checking of the css-tree library.