Port 'CSS Gradients' demo to Vala

[BharatAtbrat]

Intitial port for CSS gradients

• Gtk Style Context was deprecated (since 4.10) and needs an alternative
• Javascript and Python implementations also use the same

[sonnyp]

Needed to make a tweak to workbench cli https://github.com/workbenchdev/Workbench/commit/d8f06f1b46057506c49f0ec4a3e1a00fc018dc9e

I will merge this before the next release so CI can keep passing.

[sonnyp]

@Diego-Ivan @lw64 can one of you review ?