feat: add Typescript typings

[IlyaSemenov]

Currently, importing postgres-date in a Typescript project leads to error:

Could not find a declaration file for module 'postgres-date'. '/Users/semenov/work/xxx/node_modules/postgres-date/index.js' implicitly has an 'any' type.
  Try `npm i --save-dev @types/postgres-date` if it exists or add a new declaration (.d.ts) file containing `declare module 'postgres-date';`ts(7016)

This PR adds typings for the exported function.

Note that Inifinity / -Infinity can not be used as number literal types yet (see microsoft/TypeScript#32277) that's why I mark the return type as Date | number.

[bendrucker]

Please add some tests using https://github.com/SamVerschueren/tsd to verify that these are valid then I'm happy to get this merged

[IlyaSemenov]

@bendrucker I added tsd tests.

[bendrucker]

Perfect, thanks!