feat: Add purpose props on icons

[Sako-0384]

Just adding the feature below.
https://w3c.github.io/manifest/#purpose-member

Usage:

icons: [
  {
    src: path.resolve('./icon.png'),
    sizes: 512,
    destination: 'icons',
    purpose: 'any maskable'
  }
]

[webschik]

@arthurbergmz, could you add this fix in the next release?