chore(npm): adds npmignore file

[timbru31]

This excludes unneccessary files from npm releases.

Prior, it included, e.g., the `tests/` folder and `.eslintrc.js` files:

```sh npm notice npm notice 📦 @d2d/cordova-plugin-secure-storage-echo@5.0.2 npm notice === Tarball Contents === npm notice 4.6kB CHANGELOG npm notice 1.0kB LICENSE npm notice 1.1kB src/ios/SAMKeychain/LICENSE npm notice 7.3kB src/ios/SAMKeychain/SAMKeychain.h npm notice 4.0kB src/ios/SAMKeychain/SAMKeychainQuery.h npm notice 409B src/ios/SecureStorage.h npm notice 3.7kB src/android/AbstractRSA.java npm notice 3.2kB src/android/AES.java npm notice 2.2kB src/android/RSA.java npm notice 263B src/android/RSAFactory.java npm notice 1.4kB src/android/RSALegacy.java npm notice 14.2kB src/android/SecureStorage.java npm notice 1.4kB src/android/SharedPreferencesHandler.java npm notice 959B .eslintrc.js npm notice 3.2kB src/windows/SecureStorage.js npm notice 3.8kB www/securestorage.js npm notice 16.6kB tests/tests.js npm notice 1.6kB package.json npm notice 844B tests/package.json npm notice 4.1kB src/ios/SAMKeychain/SAMKeychain.m npm notice 9.5kB src/ios/SAMKeychain/SAMKeychainQuery.m npm notice 7.1kB src/ios/SecureStorage.m npm notice 13.2kB README.md npm notice 1.6kB src/ios/SAMKeychain/SAMKeychain.bundle/en.lproj/SAMKeychain.strings npm notice 2.8kB plugin.xml npm notice 899B tests/plugin.xml npm notice === Tarball Details === npm notice name: @d2d/cordova-plugin-secure-storage-echo npm notice version: 5.0.2 npm notice package size: 25.3 kB npm notice unpacked size: 110.8 kB npm notice shasum: d7f7740cb89079f870c14e556c926d5466730431 npm notice integrity: sha512-IqoLx+Ti/+D77[...]1XGsRxJrxDBmw== npm notice total files: 26 npm notice ```

Afterwards, it omits those files:

```sh npm notice npm notice 📦 @d2d/cordova-plugin-secure-storage-echo@5.0.2 npm notice === Tarball Contents === npm notice 4.6kB CHANGELOG npm notice 1.0kB LICENSE npm notice 1.1kB src/ios/SAMKeychain/LICENSE npm notice 7.3kB src/ios/SAMKeychain/SAMKeychain.h npm notice 4.0kB src/ios/SAMKeychain/SAMKeychainQuery.h npm notice 409B src/ios/SecureStorage.h npm notice 3.7kB src/android/AbstractRSA.java npm notice 3.2kB src/android/AES.java npm notice 2.2kB src/android/RSA.java npm notice 263B src/android/RSAFactory.java npm notice 1.4kB src/android/RSALegacy.java npm notice 14.2kB src/android/SecureStorage.java npm notice 1.4kB src/android/SharedPreferencesHandler.java npm notice 3.2kB src/windows/SecureStorage.js npm notice 3.8kB www/securestorage.js npm notice 1.6kB package.json npm notice 4.1kB src/ios/SAMKeychain/SAMKeychain.m npm notice 9.5kB src/ios/SAMKeychain/SAMKeychainQuery.m npm notice 7.1kB src/ios/SecureStorage.m npm notice 13.2kB README.md npm notice 1.6kB src/ios/SAMKeychain/SAMKeychain.bundle/en.lproj/SAMKeychain.strings npm notice 2.8kB plugin.xml npm notice === Tarball Details === npm notice name: @d2d/cordova-plugin-secure-storage-echo npm notice version: 5.0.2 npm notice filename: d2d-cordova-plugin-secure-storage-echo-5.0.2.tgz npm notice package size: 22.9 kB npm notice unpacked size: 91.5 kB npm notice shasum: 66446dafaaaeb3d94e57245ab5688ff774139ac2 npm notice integrity: sha512-OQUPIXgOSQkNm[...]Qv6hxWp18qVUg== npm notice total files: 22 npm notice ```