Issue 55/separate maskable icon file

[cameronrr]

Added ability to specify a separate 'maskable' icon.

This allows developers to create a combination of PWA icons suitable for their applications:

• Single icon, generate as 'any'
• Single icon, generate as 'maskable any'
• Separate icons, generate one as 'any' and the other as 'maskable'

Tested locally using npm link and checked generated manifest.json using manifest-validator. Scenarios tested:

{ icon: 'static/icon.png' } Expected: Manifest contains array of 'any' icons. Expected: dist/assets/static contains single set of icons. Result: As expected

{ icon: 'static/icon.png', maskableIcon: false } Expected: Manifest contains array of 'any' icons. Expected: dist/assets/static contains single set of icons. Result: As expected

{ icon: 'static/icon.png', maskableIcon: true } Expected: Manifest contains array of 'maskable any' icons. Expected: dist/assets/static contains single set of icons. Result: As expected

{ icon: 'static/icon.png', maskableIcon: 'static/icon.png' } Expected: Manifest contains array of 'maskable any' icons. Expected: dist/assets/static contains single set of icons. Result: As expected

{ icon: 'static/icon.png', maskableIcon: 'static/icon-maskable.png' } Expected: Manifest contains array of 'any' and 'maskable' icons. Expected: dist/assets/static contains two sets of icons (maskable, any). Result: As expected

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[rishabh3112]

@cameronrr Thanks for pull request! It looks great!