fix: add type declarations for serialize-request

[rowanmanning]

This adds manual type declarations for serialize-request which should make it work in most projects. The tests are now passing.

The compromise is this:

• ✅ In a CommonJS app (non-compiled)
• ✅ In an ESM app (non-compiled)
• ✅ In a TypeScript app that's written in ESM (esModuleInterop set to true)
• ✅ In a TypeScript app that's written in ESM (esModuleInterop set to false)
• ✅ In a CommonJS app which uses Sucrase
• ❌ In a TypeScript app that's written in CommonJS (esModuleInterop set to true)
• ❌ In a TypeScript app that's written in CommonJS (esModuleInterop set to false)

In the ❌ed projects above everything will still work but you won't get type hinting in VSCode and the types will be any. I'm not sure if it's possible to fix this without breaking the other module systems which are more commonly-used.