Convert macro.js to TypeScript

[JoshuaKGoldberg]

Following #50 -> #51: I didn't try to convert macro.js to TypeScript because I'm not very familiar with https://github.com/kentcdodds/babel-plugin-macros. But in theory it'd be nice for someone who is more familiar to:

• Delete macro.d.ts
• Rename macro.js to macro.ts
• Adjust the build tooling so that macro.d.ts gets recreated, akin to today's build:types for dedent.js -> dist/dedent.d.ts