update +list-themes preview demo to Zig 0.16 syntax

[tacheraSasi]

What do you want to change?

I noticed the +list-themes theme preview renders a fake ziggzagg.zig sample that uses std.Io.getStdOut().writer() and pub fn main() !void, which is not valid in any Zig version. So i want to change the hardcoded sample in src/cli/list_themes.zig to current 0.16 idioms (pub fn main(init: std.process.Init), std.Io.File.stdout().writer(init.io, &buf), w.flush())

Why do you want to make this change?

The stale demo shows incorrect Zig to anyone running ghostty +list-themes, i suppose it was left over from before the 0.16 migration (#12228) it wasnt updated in the migration

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[jcollie]

Already vouched in the PR. Thanks for opening the discussion though!

[jcollie]

#14012

[tacheraSasi]

thanks 🥲