chore(npm): improve ignore list

[timbru31]

Platforms affected

Motivation and Context

Description

Testing

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[timbru31]

Testing can be done with npm pack --dry-run:

npm notice
npm notice 📦  cordova-plugin-device@2.0.4-dev
npm notice === Tarball Contents ===
npm notice 11.4kB LICENSE
npm notice 163B   NOTICE
npm notice 957B   src/ios/CDVDevice.h
npm notice 932B   src/osx/CDVDevice.h
npm notice 5.4kB  src/android/Device.java
npm notice 3.0kB  www/device.js
npm notice 2.8kB  src/browser/DeviceProxy.js
npm notice 4.4kB  src/windows/DeviceProxy.js
--> SEE HERE  npm notice 4.5kB  tests/tests.js
npm notice 1.3kB  package.json
--> SEE HERE  npm notice 260B   tests/package.json
npm notice 3.3kB  src/ios/CDVDevice.m
npm notice 3.8kB  src/osx/CDVDevice.m
npm notice 1.4kB  CONTRIBUTING.md
npm notice 7.3kB  README.md
npm notice 11.7kB RELEASENOTES.md
npm notice 1.4kB  types/index.d.ts
npm notice 3.2kB  plugin.xml
--> SEE HERE npm notice 1.2kB  tests/plugin.xml
npm notice === Tarball Details ===
npm notice name:          cordova-plugin-device
npm notice version:       2.0.4-dev
npm notice filename:      cordova-plugin-device-2.0.4-dev.tgz
npm notice package size:  18.0 kB
npm notice unpacked size: 68.4 kB
npm notice shasum:        230a362b1c11848e96b2434ca5f0f8326be099ea
npm notice integrity:     sha512-vb64LVNUOulYO[...]5zcqnltjI3ZCQ==
npm notice total files:   21
npm notice
cordova-plugin-device-2.0.4-dev.tgz

Obviously, we shouldn't ship the tests/ folder with npm.