feat: add mercury.com theme

[dakdevs]

Summary

• adds the dark purple and light themes from Mercury's design system.

Company: https://mercury.com

[dakdevs]

@thdxr apologies for getting ahead of myself. got a little too excited 😰

[ariane-emory]

Nice lookin' theme.

I always like more themes since I like to set different themes in every project's .opencode/opencode.json so that they're easier to tell apart.