Canvas and DOM renderers should force transparency in the selection color

[Tyriar]

Downstream: microsoft/vscode#86072

The webgl renderer supports opaque colors as it's drawn behind, the other renderers draw on top due to their own limitations. Could we just force 50% transparency or do some clever blending to get the approximate color for dom and canvas?

[Tyriar]

This is already done:

xterm.js/src/browser/ColorManager.ts

Lines 146 to 153 in b11217a

	/**
	* If selection color is opaque, blend it with background with 0.3 opacity
	* Issue #2737
	*/
	if (color.isOpaque(this.colors.selectionTransparent)) {
	const opacity = 0.3;
	this.colors.selectionTransparent = color.opacity(this.colors.selectionTransparent, opacity);
	}

Only follow up for canvas: #3937