Update babel-plugin docs to hint at @babel/preset-typescript

[kitten]

Instead of setting up a TypeScript + Babel transpilation pipeline it's also possible to set up a pure Babel pipeline, as for other compile-to-JS languages now.

This has the added benefit of working the same as Flow or "vanilla" ESnext transpilation and is much easier to get right.

https://babeljs.io/docs/en/next/babel-preset-typescript.html

Related: styled-components/babel-plugin-styled-components#41 (comment)