build: convert dedent.ts and tests to TypeScript

[JoshuaKGoldberg]

Fixes #50.

Integrates with a few tools:

• TypeScript itself for type checking (yarn tsc)
• Babel's TypeScript preset to stick with the existing output process (yarn build:legacy, yarn build:modern)
• tsup to generate a single .d.ts from dedent.ts (yarn build:types)

Types are moved from index.d.ts to dist/dedent.d.ts to match the other dist/* files. I confirmed with the attw cli that these support various Node resolution strategies.

macro.js and macro.d.ts are untouched, per #50 -> #52.

[JoshuaKGoldberg]

Published as dedent@1.1.0 (#53).