Allow theme extensions to implemet settings for diff tweaks

[soymadip]

• Themes & theme extensions should have settings to change diff aspects.
• This allows us to more personalize our zed setup to our preference.
• Specially accent color option would be very helpful.
• Like for catppuccin theme we have:-

[soymadip]

Like an user could just use something like in pic to change accent:-

{
 "theme-name.accentColor": "lavender",
}