Support 8-bit palette grayscale values

[ian-h-chamberlain]

According to the Wikipedia article linked in the README (and from a quick test in my terminal), valid values for the 8-bit color mode include the range [232,255] for grayscale values, but it seems the library doesn't support this. use_classes: true seems to still work as expected, so it really just seems to be the conversion from palette color to RGB that is not supported:

var anser = require("anser")

const txt = "\u001b[38;5;255mHello\u001b[39m \u001b[48;5;235mWorld\u001b[49m";

console.log(anser.ansiToHtml(txt));
// <span style="color:rgb(undefined, undefined, undefined)">Hello</span> <span style="background-color:rgb(undefined, undefined, undefined)">World</span>

console.log(anser.ansiToHtml(txt, { use_classes: true }));
// <span class="ansi-palette-255-fg">Hello</span> <span class="ansi-palette-235-bg">World</span>

Converting to JSON instead produces similar results.