feat: support 8-bit color

[pd93]

Adds support for multiple ANSI attributes and 8-bit colours when specifying TASK_COLOR_X variables. Pairs nicely with the functionality in #1572.

You can specify multiple codes by separating them with a semicolon. For example: 31;1 will make the text bold and red. You can specify 8-bit colours by using the sequence 38;2;R;G;B for foreground colours and 48;2;R;G;B for background colours where R, G and B should be replaced with values between 0 and 255.

For convenience, you can also specify 8-bit foreground colours using a shorthand, comma-separated syntax: R,G,B. For example, 255,0,0 is equivalent to 38;2;255;0;0.