tns resources generate splashes : use CSS background property values for the --background option

Is your feature request related to a problem? Please describe.

When generating splash screen with the tns resources generate command I cant set a linear gradient background for example. The background is always one color, thus restricting the possibility of customizing our app.

Describe the solution you'd like

Something like tns resources generate splashes <Path to image> --background 'linear-gradient(135deg, rgba(7,10,61,1) 0%, rgba(18,51,127,1) 100%)'
The --background option can take the value of the CSS background (or background-color at least) property

Describe alternatives you've considered

Or add a --style option that can take the path of a css file that contain the property of the splash screen.

Additional context

None.

从 tns resources generate splashes 命令的实现入手，跟踪 --background 选项如何被解析并应用于生成的 splash 资源。检查 Android 和 iOS 现有的 splash 生成行为，然后验证受支持的 CSS 背景值能够生成预期的资源，同时不会破坏纯色背景。