Support web manifest icon for "purpose: monochrome"

[jahed]

There's currently a "manifestMaskable" option available to create "purpose: maskable" icons but there's no equivalent "manifestMonochrome" option to create "purpose: monochrome" icons.

Favicons should generate this icon too for a complete set of web manifest icons. https://developer.mozilla.org/en-US/docs/Web/Manifest/icons

This could also be used to generate Safari's mask-icon. https://developer.apple.com/library/archive/documentation/AppleApplications/Reference/SafariWebContent/pinnedTabs/pinnedTabs.html