Add missing type export conditions and index.d.cts

[AtoraSuunva]

While the library has types, it's missing the proper conditional types exports in package.json that causes any project using node16 or bundler module resolution in Typescript to fail to build:

Could not find a declaration file for module '@dice-roller/rpg-dice-roller'. '<project>/node_modules/@dice-roller/rpg-dice-roller/lib/esm/bundle.js' implicitly has an 'any' type.
  There are types at '<project>/node_modules/@dice-roller/rpg-dice-roller/types/index.d.ts', but this result could not be resolved when respecting package.json "exports". The '@dice-roller/rpg-dice-roller' library may need to update its package.json or typings.

You can also see the types failing to resolve here:

This PR adds in the missing types condition to exports (and a required index.d.cts that's just a copy of index.d.ts) so that Typescript is happy and can resolve types and compile:

Note that you'll still need skipLibCheck: true in tsconfig.json regardless of your module settings until #247 is fixed though.

[GreenImp]

Looks great, thanks!

[GreenImp]

This has just been released in v5.4.1