fix: add package.json to exports for react-native-cli

[leezumstein]

Context

This resolves #184 .

Recent updates to the project were causing warnings to be thrown because react-native-cli couldn't resolve the package.json file. This came down to the file needing to be added to the exports for the module so it could be properly resolved. Example of how react-native-cli is loading the modules for validation here, with the important bit being here where it resolves the package.json file from the module.

Steps to Reproduce

1. Setup empty repo

   yarn init -y
   yarn add -D axios-retry
   touch index.js

2. Add the following content to the index.js file to test the error

   const path = require("path");
   
   let moduleDir;
   try {
    moduleDir = path.dirname(
      require.resolve(path.join("axios-retry", "package.json"), {
        paths: [path.join(__dirname, "node_modules")],
      })
    );
   } catch (e) {
    console.error("unable to locate module dir, reason:", e);
    return;
   }
   
   console.log("found module dir:", moduleDir);

3. Run the following and confirm an error is logged with the following

   node index.js

   unable to locate module dir, reason: Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './package.json' is not defined by "exports" in node_modules/axios-retry/package.json
      at throwExportsNotFound (internal/modules/esm/resolve.js:285:9)
      at packageExportsResolve (internal/modules/esm/resolve.js:508:3)
      at resolveExports (internal/modules/cjs/loader.js:424:36)
      at Function.Module._findPath (internal/modules/cjs/loader.js:464:31)
      at Function.Module._resolveFilename (internal/modules/cjs/loader.js:802:27)
      at Function.resolve (internal/modules/cjs/helpers.js:80:19)
      at Object.<anonymous> (/Users/leezumstein/dev/forks/test/index.js:6:13)
      at Module._compile (internal/modules/cjs/loader.js:999:30)
      at Object.Module._extensions..js (internal/modules/cjs/loader.js:1027:10)
      at Module.load (internal/modules/cjs/loader.js:863:32) {
   code: 'ERR_PACKAGE_PATH_NOT_EXPORTED'
   }

4. Apply the change to the exports in the package.json present on the PR
5. Re run the following and confirm the error is gone

   node index.js

   found module dir: <resolved path to axios-retry>