Added packageExists

[jdgjsag67251]

Use case

We would like to know if we are able to start a specific intent before actually calling it. The solution I came up with was checking if the package that exposes the intent is available on the system.

Example

const packageName = 'com.android.contacts';

window.plugins.intentShim.packageExists(packageName, (exists) => {
    if (exists) {
        console.log(`${packageName} exists!`);
    } else {
        console.log(`${packageName} does not exist...`);
    }
});

[darryncampbell]

Reviewed and all looks good, thank you for your contribution.

[darryncampbell]

Posted to npm as 2.2.0