fix: support modern esm and types resolution

[IlyaSemenov]

Currently, the package won't run with vite/rollup which is arguably one of the most (if not the most) popular bundler setup.

First, it would use a CJS build instead of ESM build even in pure ESM ("type": "module") projects. This is because the ESM build is not published in a standard-compliant way.

Then, with CJS build, it will not import sub-deps properly:

function JWTServerToken(apiSecret, jwtOptions = {}) {
    const payload = {
        server: true,
    };
    const opts = Object.assign({ algorithm: 'HS256', noTimestamp: true }, jwtOptions);
    return jwt.sign(payload, apiSecret, opts); // <------- here it will crash
    // because jwt is { default: { sign: ... } }
}

Then, under some circumstances the types won't load with modern Typescript setup ("moduleResolution" set to "Node16" or "Bundler") with this error: ...There are types at '/......./types/index.d.ts', but this result could not be resolved when respecting package.json "exports".ts(7016)

All and all, the project's package.json is missing the exports section which modern packages should have. The PR adds one.

Also, the top-level module property should be removed from package.json as there is no such field officially. It could have been supported privately by some bundlers but now everything supports exports. I kept module in this PR as you may have been relying on it somehow—but please consider removing it.

[szuperaz]

Thank you for the contribution