Support rgba format in css.toColor

[Tyriar]

VS Code issue: microsoft/vscode#149867

xterm.js/src/common/Color.ts

Lines 96 to 110 in 87bc48f

	export function toColor(css: string): IColor {
	switch (css.length) {
	case 7: // #rrggbb
	return {
	css,
	rgba: (parseInt(css.slice(1), 16) << 8 | 0xFF) >>> 0
	};
	case 9: // #rrggbbaa
	return {
	css,
	rgba: parseInt(css.slice(1), 16) >>> 0
	};
	}
	throw new Error('css.toColor: Unsupported css format');
	}

[Tyriar]

Didn't do the alpha channel right, rgba(0, 0, 0, 0.5) will fail