Allow for custom icons for non-file based editors (extensions, keybindings, search, settings)

[Tyriar]

Nice easy polish change to switch to the 4 square icon here:

[Tyriar]

Makes sense for other editors: keybindings, search, settings

[Tyriar]

I was hoping to do this myself and looked into it, but there's no clear way other than CSS with !important which is a hack to assign an icon for. I'm also a little worried I'll cause regressions with how this interacts with icon themes which I have minimal knowledge on, so assigning to respective owners.

For reference the terminal does it by generating CSS in JS and injecting it such that the codicon chars (which could theoretically change) are not hard coded in CSS:

vscode/src/vs/workbench/contrib/terminal/browser/terminalService.ts

Lines 1246 to 1252 in 97082b0

	const iconClasses = getUriClasses(instance, colorTheme.type);
	if (uri instanceof URI && iconClasses && iconClasses.length > 1) {
	css += (
	`.monaco-workbench .terminal-tab.${iconClasses[0]}::before` +
	`{background-image: ${dom.asCSSUrl(uri)};}`
	);
	}