C4IA: Add new Chat color tokens

[aagonzales]

These tokens are needed for the AI Chat design. Not sure if this goes into the core package or the C4AI package

Token name	White theme	Gray 10 theme	Gray 90 theme	Gray 100 theme
chat-prompt-background	White	White	Gray 100	Gray 100
chat-prompt-border	Linear-gradient: Gray 10 100% to 0%	Linear-gradient: Gray 10 100% to 0%	Linear-gradient: Gray 90 100% to 0%	Linear-gradient: Gray 90 100% to 0%
chat-bubble-user	Gray 20	Gray 20	Gray 80	Gray 80
chat-bubble-agent	White	White	Gray 90	Gray 90
chat-bubble-agent-border	Gray 20	Gray 20	Gray 70	Gray 70
chat-avatar-bot	Blue 60	Blue 60	Blue 50	Blue 50
chat-avatar-agent	Gray 100	Gray 100	Gray 10	Gray 10
chat-avatar-user	Gray 100	Gray 100	Gray 10	Gray 10

[github-actions]

Thank you for submitting a feature request. Your proposal is open and will soon be triaged by the Carbon team.

If your proposal is accepted and the Carbon team has bandwidth they will take on the issue, or else request you or other volunteers from the community to work on this issue.

[tw15egan]

Just one note, the chat border has been broken into two tokens:

$chat-prompt-border-start
$chat-prompt-border-end