feat: splash screen support

[kevinmarrec]

Follows @xlanex6 work (#4)
Resolves #3

This PR will be squashed and merged with the following commit description to keep credits on @xlanex6 :

Co-authored-by: Alex Duval <[email protected]>

[kevinmarrec]

@xlanex6 You can check my changes, I think it's ready. I finally added back the splash.backgroundColor option, as well as targetDir (default: splash).

You can have a try and give your feedback :)

Also don't hesitate to ask anything regarding my changes.

EDIT: I also added tests that check splash screens are correctly generated.

[xlanex6]

@kevinmarrec
Funny that you back to my initial idea to make a root options splash with all options into.
You make it better way than I could. thank you

[kevinmarrec]

@xlanex6 Yeah I got confused with some of my thoughts at some point. You had it in the good direction mate 👍

[xlanex6]

@kevinmarrec

Are you plan to make a DOC site , with exemple , video demo to explain How it works , etc...
If so , and still possible , I will love to be part of it.

[kevinmarrec]

@xlanex6 I don't think I'll make one, because overall it will land in https://github.com/nuxt-community/pwa-module as official module. I'm working at NuxtLabs so it will be just about time that things get more official. Therefore the documentation will be on the official module :)