[gatsby-plugin-manifest] Auto generate normal icon and maskable icon

[mwskwong]

TL;DR Add an option iconMaskable (or if you have a better way to implement that, feel free to suggest) such that the plugin can support auto generating 2 sets of icons, a normal set, and a maskable set.

Currently, if you wish to have both maskable and normal icons, you have to manually generate one of them. Yes, you can instead specify purpose: "any maskable" in the icon_options, but this is a bad practice as the resultant icon will appear to have either too little or too many paddings.