Add monochrome icons for Android

[lnhrdt]

Besides any and maskable there is another manifest icon purpose called monochrome.
https://developer.mozilla.org/en-US/docs/Web/Manifest/icons#values

On Android, monochrome icons from native apps are used by Material You with OS level theming. It seems that integration with PWAs isn't quite there:
https://stackoverflow.com/questions/74620798/material-you-icon-on-pwa
But there's an open issue in Chromium:
https://bugs.chromium.org/p/chromium/issues/detail?id=1468845

I'd like to provide a monochrome icon in my manifest to:

1. make it easy to test their adoption on different platforms
2. be ready when support lands on different platforms

I'd be interested in support, similar to what was introduced in #38 for maskable icons. Or perhaps a more generic solution that allows the user to provide icons for any purpose (e.g. any, maskable, monochrome, some-new-spec-in-the-future, etc).