Fix SGR direct-color parsing issue

[qwerasd205]

Fix for a little parsing issue I took note of here.

The disparity in behavior between ghostty@main and xterm can be seen with this reproduction script:

printf "\e[0m\nForeground:\n";
printf "\e[38:2:0:255:0mGreen\n";
printf "\e[38;2;0;255;0mGreen\n";
printf "\e[38:2:0:255:0:255mMagenta\n";
printf "\e[38;2;0;255;0;255mGreen\n";

printf "\e[0m\nBackground:\n";
printf "\e[48:2:0:255:0mGreen\n";
printf "\e[48;2;0;255;0mGreen\n";
printf "\e[48:2:0:255:0:255mMagenta\n";
printf "\e[48;2;0;255;0;255mGreen\n";

printf "\e[0m\nUnderline:\n";
printf "\e[58:2:0:255:0m\e[4mGreen\n";
printf "\e[58;2;0;255;0m\e[4mGreen\n";
printf "\e[58:2:0:255:0:255m\e[4mMagenta\n";
printf "\e[58;2;0;255;0;255m\e[4mGreen\n";

printf "\e[0m\n";

Outputs:

xterm	ghostty@main	this PR

[qwerasd205]

Oh oops, I somehow left a test failure in, I must have accidentally hit undo at some point!

[mitchellh]

We should extract this to a comptime-based helper (so it can take the union tags). But this looks good and I'll merge as-is for now. Thanks!